Low-key ad hominem fallacy says "What?"
I don't play with a mouse. I play with a controller. I can fit 16 buttons on a crossbar set, 32 on two. 30 buttons is 3 times 10, which is A LOT OF BUTTONS. It's a third of the way to 100. So the idea that Jobs need 35+ buttons or are "too simple" or "people not putting effort into their gameplay" is just stupid. A lot of games limit you to around 8-16 abilities, and those aren't accused of being too simple or of players being lazy. Just stop.
For me personally, about 29 buttons is where we should draw the line. That allow for Sprint, LB, and Potion to fit in 32, and 29 is still a lot of buttons. I'm not sure why "29 is too few...but 35 is just right" comes into play, as that's a difference of only 6 buttons.
PLD, SCH, and AST are CONSTANTLY getting complained about for having too many buttons. If you aren't seeing it, I'm guessing you don't read many people's posts on the topic over the last couple years. They aren't the only ones, but seem to be the most noted offenders.
I do agree not every Job is for everyone, though. But there also is an issue of, as the game has more expansions, new abilities are added. There comes a point where it's just too many buttons for ANYONE. Where that is depends on the person - for me it's 29, for someone else it may be 40 - but if we don't see some reductions/combinations in rational and intelligent ways, eventually we surpass whatever that is.
On controller, binding all single-target things (which is especially important for NIN due to burst), requires 17 keybinds, meaning it's impossible to fit them all on one hotbar set. And while one might consider having a "burst" and "downtime" hotbar set, a good deal of the buttons used in burst are also used in downtime (Mudras and Ninki spenders, for example), meaning that would require a lot of duplication to achieve, which then would require a THIRD hotbar set to fit in the rest of the buttons. Something I used to do in SB on PLD, btw. And it was every bit as annoying as it sounds.
Single target rotations should consist of around 16 buttons, more or less. Sixteen is not a massive shortage of buttons. Really think about how many that is. And that's the "active use all the time in combat and/or during burst", not including situational stuff like debuffing the boss (Feint), AOE rotations, or other situational stuff like Shukuchi or Hide or Arm's Length.
Add to that you're generally going to have another 4-6-8 buttons for AOE, and that many more situational abilities, and that gets you around 30 total buttons, which SHOULD be more than enough for most people and most Jobs.
You like 50+ buttons, that's great. But it's generally not good game design.
It also really is a keyboard vs controller thing, since I remember playing WoW with Keyboard and having 1-5, shift, control, and alt modifiers to those (20 altogether), 6-= (8 more buttons) of situational stuff, and then other hotbars on the side for professions (given, you didn't swap into those like FFXIV), mounts, foods, and utility stuff for the class like Mage teleports or Druid forms and such. But, it was frankly ridiculous having roughly 50 buttons across all my hotbars, and only around 15 of those were routinely used in combat, with another 5-10, depending on class, being highly situational. At least it made sense there, since WoW was more of an adventure game and I didn't need "Cooking" or "Eyes of the Beast" in an easy to reach button since...you know, I wasn't using that during my burst phase in combat.
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It is fine if some Jobs have more. I'm not saying none can. But they should actually have value and justify their place on the hotbar slot, and you should really have down around 16 or so for high use and burst. That's a lot of buttons if you actually think about it. 1-0 and then 1-6 again.
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It'd be one thing if I was asking for some huge change like "get rid of Ninki and all spenders" or "get rid of Mudras", but this is one button that does make logical sense to combine, not a great culling/gutting.
That said, my OP was just asking if there were any good reasons NOT to do it...and as far as I can tell from the replies, there aren't, and some others think the same idea has merit. That answers my original question.
So thank you to all the people who responded.
And yeah, basically agreed on this.