Not to disagree with you, but I think its worth pointing out we "sort of" have an idea what they were thinking, based on direct information leaked, and indirect information leaked. (Regarding BLU, before it was released)
BLU was meant to come out with SAM and RDM for SBs release. (As a full job)
BLU was meant to melee with a weapon, and was not categorized as a Caster DPS.
After SBs release
BLU had been made as a Magical Melee (the 1st in its sub role to exist, as we only have physical melee sub role)
BLU was being delayed further, to add far more spells to it.
Near the end of SB
BLU was being delayed even further for an unknown reason to the leaker.
Leakers last posts involved telling us about Adam and Eve (Omega final bosses original names before localization) GNB, DNC, Viera, and Ronso.
Going by this information, its easy to make a guess, as to what they were thinking, and might be thinking in the future. I agree we cant know for certain, but I feel its not too hard to fill in the gaps.
So I can disagree with you on how accurate we could guess, but I can also agree with you, that his guess seems really off base, and not aligned with prior information we have.
I'll list some of my personal thoughts I've collected, to find "patterns" in SEs behaviors, in which I try to guess what they are thinking.
Besides the leakers info, we knew Yoshida mentioned only making female viera, and male "beastmen". When Viera was 1st shown, I mentioned how other races showed 1 male and 1 female, while the viera artwork showed multiple females. I told others this was more than enough evidence to know they would be female only, and people argued with me, that i couldnt know what SE was thinking. (Over simplifying this example, as a lot more happened)
Going a bit further into "assumption" territory, 2.0 designed most DPS to just be a mishmash of most basic DPS mechanics. Every DPS pretty much shared the same mechanics, except cast bars, and positionals, which were given to melee/casters respectfully.
Only MNK was considered "the complex DPS" job.
3.0 came out, every job gained more complexity. IMO, it was to discourage people flocking to just the DPS role, based on "taking it easy". But why did I think this way? Because MNK barely changed. But others like DRG and BLM grew far more difficult. You also had cast bars thrown on BRD/MCH, to bring them in line with the other ranged DPS, but mostly to add to their difficulty, due to players flocking to BRD in 2.0, to make fights easier. (not just for the support buffs, but due to ease of playing being mentioned by yoshida) Even with the tanks, WAR barely changed, but PLD really changed, and PLD was mostly known for being too simplistic in 2.0, just spaming 123 all day.
Where is this "history lesson" going?
Sorry, im slowly getting there...
well, BLU was being designed in 3.0. With SAM being the 1st, and RDM being made after SAM. This made BLU last on the list to be made. (And was worked on before SB)
All SB jobs were streamlined, and made less clunky, and somewhat easier than 3.0. SAM felt roughly on par with the general difficulty of other DPS jobs in SB (not as punishing as 3.0 jobs, but equal to their 4.0 versions) So if SAM felt roughly equal to the 4.0 versions of DPS, why was RDM so simplistic? Its the only DPS who doesnt have their primary mechanic as a "Timer that ticks down".
I assumed it was SE saying, we're going to just make 1 DPS more simplistic than another. (I feel this is wrong, and will explain why later)
Now for Limited BLU... Yoshida brought up defenses for limited BLU, including the idea that BLU would have broken abilities to instantly kill bosses. And he HAD to give BLU these abilities, because it just wouldnt be BLU w/o them. The initial statement proved to be misleading right off the bat. And the second statement just doesnt make sense.
I assumed something was up at this point, but didnt know what yet. Yoshida has always covered up mistakes with these sorts of "salesman" white lies. Similar to how he tried to falsely justify why they gender locked the two races. (He already explained the reasoning for it many years ago, but people didnt like his justification, so he changed the narrative, so more people would accept the outcome.) I'm not saying he's wrong for doing this, I'm just pointing out it happens.
DNC is announced, and its trailer looks like it has cast bars, with familiar healing animations, and has physical ranged attacks. People were confused which role it was, and after yoshida answered, he even mentioned he wasnt sure if it was getting aiming or scouting gear yet. (Which seems odd to not be sure of this late, but that sort of thing can happen)
GNB also seemed odd, as its a whole job based on a weapon, rather than a "job". But nothing sus about it though.
Finally 5.0
Tanks and healers are nerfed into very simplistic designs. This seems to go in hand with my theory on why they raised DPS difficulty in 3.0, which is to encourage certain players into playing more tanks/healers, based on "ease".
NOW TO THE POINT FINALLY
My theory is as follows;
during heavensward, they planned out the next 3 jobs. SAM was melee DPS, and RDM was actually planned to be a Healer. Their DPS rotation would be very simplistic and easy to follow as a healer, as theres no timer ticking down.
And BLU was going to be a Melee Caster Tank. (Maybe what inspired PLD to get cast bar nukes)
WHY do I think that? Thats a VERY long story, but the simplistic version stems from how FF14 treats AoEs (BLU being one of the few AoE based jobs in FF history) BLUs need to get hit by spells in some games, plus 2 other factors that came up later. *
I think they didnt have time to finish BLU, and last minute changes to RDM slowed BLUs development further.
(Touching on RDM healer again)
I think one of the main reasons RDM became a DPS, was to include its melee combo. Which I think it has a shorter GCD solely to help it get back to healing faster, as a caster DPS doesnt need its burst phase to be 1.5s GCD. Its just comes off as random flavor. It also doesnt really regenerate MP naturally in its own kit, like the other casters. Its more like a healer, who tries not to run out of MP before the fight ends.
I think they planned to change tanks to be the "5.0 simplistic " design, as they were working on BLU, which lead them to make it into a DPS instead.
Which weapon did they give it? Why does BLU mimic FFXIs designs of; "See an ability" (not get hit by it) and "Equip spells into slots"?
I think they took its weapon, and animations away, which is why it got delayed so much, and was given even more spells later on. They turned it into a caster, and repurposed the "Tank" side of it, and its melee animations.
I noticed the GNB shells glow blue, and the original AF artwork design was Blue. * This is of course is a huge jump, but it would make sense why BLU has to "Equip" spells like FFXI, as it probably had to be assigned a slot in the "chamber" of the GNB weapon. If BLU isnt going to tank, it cant be hit by the spell anymore. If GNB is the only job we see using a melee weapon, its not too unreasonable to assume its the job that got the removed BLU weapon/animations. (Some GNB animations even show the weapon shooting, despite yoshida saying the weapon doesnt, because its FF8s gunblade)
The additional back up to this theory, is me also theorizing how DNCs storyline is about healing people, and why they have their odd heal. The general animations of some abilities, along with how the dance itself also nukes once done. If DNC were to be a healer, the dances would take up too much of your time needing to heal in some cases. so attaching a potent AoE heal that triggers once u do the dance, covers your bases pretty well. Also it would have to have ranged attacks. its DPS rotation would have to be pretty simple. But the part thats not accurate to healer, is that it has a buff timer, which ticks down... but then again, you can do your dance extra early, to refresh it... so its not TOO hard to do... right? Well healers difficulty got nerfed... so theres NO way such a healer could exist, and I think they wanted to keep what they made, and just go with a physical ranged DPS. This might explain why we went 2 expansions w/o a healer, and why yoshida wasnt sure on some of DNCs stuff, even as late as its official reveal.
Now, if BLU is too complex to be a tank, and GNB gutted BLU, it would make sense that they decided they could use the corpse of whats left of BLU, and "make a limited job".
This would explain all the odd decisions made about BLUs "limited" status, that clearly go against the very rationale that yoshida claims.
Am I right? I have no clue, I might be, I might not be, or I may only be half right. But this little example shows how its possible to "get in their heads" to some degree, and with the viera example, I was able to predict an outcome based on prior patterns. I may have as well with BLU, but I'll never know. But this explanation is also why I think BLU will eventually be made into a full job. (There are other reasons i believe as well)