The thing is, time is their mortal enemy. So regardless of the feasibility of things, they have quotas to meet and they will do that which they perceive will help the health of the game more and weigh the development costs & time spent vs projected gains. They're developing the game all the time, just not in the way you want.
Of course it's not. But again, they've made it clear time and again that they desire balance in their level sync system, and so they're not going to do anything until they have a system that works exactly the way they want while making sure there's no issues, weighing all the cons & pros, all the things that would need balancing, create expected deadlines & time spent vs projected gain numbers. If I, a simple player that has only taken a couple courses on rudimentary software design because she was bored outside her normal courses in university, can point out all the glaring flaws and the sheer amount of things they'd need to modify/change/edit/add/remove/etc in your system just to make it work compared to their current extremely simple system, their actual experienced game designers will have long since either tossed it out, or be looking elsewhere for a simpler idea.
Except that simply divides the queues; which need I remind you, are designed to fill parties as fast as possible; another reason why it'd never see DF use without being mandatory. And I seriously doubt they'd ever do such a giant overhaul in the first place just to make it a PF only option. As far as the first bit, again; I'm simply pointing out every possibility, which by their job, a game design has to account for. There's a reason the #1 rule in software design is "plan assuming your end user will do anything and everything to break you system, whether through ingenuity or ignorance." Without taking in EVERY factor, EVERY possible outcome, any system they implement will just as easily crash down on them.
At least in their current system, High level player's skill is kept in check by the limited toolkits.
Which again, comes back to the MNK scenario; Given that jobs learn different parts of their toolkit at drastically different levels, you're essentially asking for them to go through every job at every level to determine what part of their toolkit they have access to, and then hit them with % reduction based on that. Since -20% on a BRD, who barely gets anything pre-50 will be vastly different than applying that same -20% to a SAM who gets a lot of powerful potency attacks at the same level.
If your solution can't be solved with a universal multiplier for every job at every level to create the same balance as currently, it's already less precise and far more complicated than what they have now.
Go pull every pack up to the first boss of Tam-Tara with 5 completely random healers without dying once, take vids, compare that to pulling 5 packs in Mt Gulg and then tell me it's "no more than a GLD could do." Or every single mob up to the first boss of Halatali with the same requirements.
Also people kick for even less reasons than that. I've seen tanks get kicked for single pulling, you really think people aren't above the possibility of kicking for a better tank? Plus, unless you're queuing in off hours, tanks/healers almost always get insta-refreshed anyway. (I've personally never waited longer than 20s for a tank to come in during the couple of times a tank was kicked in my party.)
^
Also, You chain CDs. Once the tank leaves Hallowed, he should have a 30% rolling on up so that he stays buff and not just instantly drop dead or becoming weaker. 10s of 0 damage is a huge advantage gained, because it gives the healer 100% uptime during the 10s; 10s of knowing that whatever the healer does, you will not drop dead.
The average mob does about ~28 damage in Sastasha out of a tank's ~450 health. In our current system (or about 84 damage per pack of 3), an AST can get that much uptime due to their OGCD alone and because most tanks don't pull much more than 2 packs in most cases. An AST isn't keeping anywhere close to 100% uptime in sastasha when the tank is losing 1/3rd-1/2 their health every GCD going past the 2-pack mark with their OGCD on a 30s timer.
Which again, requires way more investment than you realize, as from a pure math standpoint, is extreme more effort than other solutions which require immensely less effort.
But a 20 GLD can't use either, so they do count against it, you can't simply ignore that mitigation difference when it is in fact a + to the PLD's toolkit. Clemency may be a DPS loss, but if it saves the pull, its a dps gain, hence why its a tool for the PLD. (Such as saving yourself on the Mt Gulg mega pulls if the healer falls behind a bit - staying alive and losing a GCD of damage is infinitely less valuable than staying alive to complete the mega pull which saves literal minutes.)
As for the last point, then, again: what is the point of halving/quartering/whatever these skills down and butchering them, when the whole purpose of your system is to allow 80's to use their full toolkits? If you have to edit and butcher skills just to give you system even the slimmest chance of maintaining balance, you're no longer playing the exact job you had at 80, merely a gutted version of it. I really can't explain this any clearer.
The major difference here being that a 75 & 76 WAR have access to 99% of the same toolkit as each other, so the disparity is far, FAR less than a 20 MRD who not only has 6 less CDs than a 76 WAR, but also 0 lifesteal. And I could repeat myself on the 2nd part, but really. Even if you were to quarter the healing of it, it's still > 0, meaning the 80 WAR would still have literally infinitely times higher HPS than a 20 MRD...or any <76 WAR in AOE, for that matter. Also again, having to butcher a job's identity just to give your system an even slim hope of being balanced.
Because they never had to factor them in early dungeons when designing them? Literally the earliest non-trick attack Raid buffs are at 50 (Divination, Battle Voice). All of a sudden in your system though, you've got 80's hopping in with all their raid buffs. let me tell ya how fast you can annihilate packs with BV + Litany + Divination. In your system, they are now an issue that has to be factored in.
For no reason? Because its literally the design the devs have been wanting to achieve with their sync system. you can't get much tighter than "everyone has access to the same skills outside of some scenarios in the early levels, which only really apply to tanks because healer & dps role actions are just fluff and not giving extra power to the healer's healing or dps's damage." With their system, a Dev can reasonably know that a group in halatali isn't going to be super mitigating enemy damage, throwing out raid buffs or massive swathes of multiple AOE GCDs & OGCDs to melt packs, or commit 100% of their healing to regens & OGCDs. In their current system, player skill variance is the only thing they can't account for; an issue that is magnified and needs to be much more directly addressed in your theoretical system due to the plethora of unexpected variance it can cause.
Let me answer that for you. "Server Limitations." "next!" Yoshi-P has long since realized making promises in public often ends in backlash, so pretty much any feature they're not personally doing will be met with a PR-friendly/neutral response; hence why it's easy to use past development cycles and their general response to suggested things to quickly answer your own question.
You can have your opinions, but observations of their dev cycle and implementations give pretty good concrete facts about how likely a potential idea would be accepted. Just from my courses in software design and general observation, I'm giving an observed opinion based on those realistic facts. They have a very 'forward looking; only fix the old when necessary.' Approach to their game design.
It took them literally 6 years to fix ARR and they still have Hrothgar/Viera to deal with, along with designing all the usual content each patch, especially now with an impacted dev cycle due to COVID. The odds of them doing anything to the level syncing that isn't as simple as 'shift a single variable on the skills' is basically null.