I actually know pretty well how this feels--I'm not a charismatic personality by nature but I have always been highly creative, and that has led to me developing many ideas with very specific goals in mind only to have someone else take over and spin things off into a wildly different direction so that the end result doesn't look anything like what I intended--and sometimes it has even still bore my name and been rather embarrassing since it's no longer really mine and not something I'd necessarily stand for.
But here's the thing: I was a kid.
Yes I know what it feels like, but I also know the telltale signs of when it's going to happen, and as an adult I'd either clear my name of the project and let people do what they want or stand my ground and maintain control. Now if you throw in business politics this may or may not be possible, and if not then yeah, it would be a frustrating place to be. But it's also expected. If you're in any type of creative industry and can't handle having your babies raised by other people, you probably won't last long in the industry. When you make an IP that gets big enough, you sell it. You sell it to a company who will let you have influence over it if possible, but you better be prepared for them to leave you behind. And when that happens you better be prepared to pick up some other idea you've got and pursue that instead. That's just the way the industry works, and it's not necessarily as evil as it sounds. You talk a lot about Tetsuya Nomura, but what about Hironobu Sakaguchi? He's the father of the entire series and watched it drift further and further outside his guidance. But at the same time, if he had remained in control of the series, would we have Final Fantasy XV or Versus XIII at all? Probably not.
As a creator, your goal should be to create IP that attracts fans who can rise up and carry on the IP without you. That's the only way to give your IP true longevity.
Now, as for Nomura, the situation with him and FFXV is not as depressing as you think. He even said himself that he was surprised to see fans acting like Square Enix booted him off the project. When it was decided to switch Versus XIII into a main series title, Nomura did the ground work and then left to focus on Kingdom Hearts III. He had a job and finished it. I'm sure FFXV has evolved greatly since the work he did, but Nomura's not the martyr you're making him out to be on this one.
This, this, and sooo much this. Being an illustrator, I myself also know what it's like -as an adult- to have to deal with this. And as @AuronX said, as an adult it's not the same... if you're actually acting like an adult. Are there times where you can feel screwed? Yeah. Can it kinda suck sometimes? Yeah. But it's also your own fault if you're not prepared in some way.
There are three core problems with turning Nomura into a martyr:
1. He should know how this works already, and know that such removal is a possibility. So he should prepare his mind for the idea that he might have to give up some control at some point. This is not at all an uncommon thing in any entertainment industry. And guess what... 90% of the time it ends up just fine, because everyone involved is an adult. A lot of the time it's even planned from the beginning, because of conflicts in time management (kind of like now, with Kingdom Hearts stuff also needing work done). The rare controversial version of this happening seems much more common, because it's the only news worthy one that people talk about. But that's not actually the case. It would be like only ever seeing tigers in your life, and assuming they must be the most plentiful animal on earth, despite other people telling you that they're endangered. Stupid example right? Lol.
2. If he's a real team player, he should know that this isn't ever solely his vision. Nomura is not a God. Sakaguchi is not a God. Kojima is not a God. They can only get something great done, with the skill of many, many other people. They understand this. These projects are not their vision alone. Adding one person to the mix doesn't always change much, even if they're in a leadership position. Why? Because the team is just as important as the leader.
3. A truly great leader builds his/her team up to work great without him/her. In working on a team, you should always be striving to get everyone to see a unified, passionate vision. When one person has a weakness, someone else needs to make up for that weakness. The best way to make a great product, is to build everyone up to be great. Most of the time, this is the only way to make a great product. So a good leader will usually end up with a team that can make a great product without them.
As everyone has said multiple times, there is no way to know what the game might have been in other circumstances. Realistically, the information we had for the 'old version' of XV was basically nothing. It's really strange to me that you're forcing yourself to get so upset by stuff you know nothing about. You're ruining your own outlook on this game, based on what? A few pieces of rumor or early ideas that don't make it in? If only you knew just how much changes in the development of a game. They're rarely anything like the early visions, or even the mid development visions. You don't know whether Nomura's vision was really solid or not anyway. Out of all this game is shaping up to be, we maybe had access to about .01% of all the plans they had. That's basically nothing. We simply don't know how the game would have ended up. But most importantly, we don't know what state the game will actually be in when it's finished. It could be absolutely fantastic. Are you really willing to ruin your experience with it, just so you can cry over the loss of someone who doesn't even seem to be crying themselves?
Evidence dictates that the situation is the opposite of what you want to assume. The split seems to have been more than amicable, and all of Nomura's work was purportedly already done. The game seems to be in great shape. And as I said up top, Nomura is notorious for over-working concepts when he's in a leadership role the entire time anyway, meaning even from a what-if perspective, it's easier to argue this is a good thing than a bad thing anyway. So why would anyone assume that this is a bad thing? Why worry over a situation that seems to be the opposite of what's actually happened?
By asking such a question as you are, and complaining essentially about nothing, you're devaluing the work everyone else on this team has done; you're inadvertently giving Nomura credit for basically everything positive in the creation of this project. If you really wanted this project to succeed, you would first and foremost support Tabata and the rest of the team, not worry about Nomura's feelings. He's a big boy. So if I were to answer your question... I would feel fine if I was in his shoes. I've been in his shoes, I'm an adult, and I know to expect this sort of thing. I don't have teams work for me. I work with teams, in the hopes that if I have to step back, they can make a great project without me. I respect their skills, and know that I'm not the center of the universe.