I guess that "scrappy indie" chaos only works when you have the freedom to iterate
They were never a "scrappy indie". Bioware was an AAA company (for the time) since day 1, and Bioware was owned by EA for the vast majority of the time it put out what you call "legendary RPGs".
Further, those people didn't quite when things "got too corporate", they quit when the games started failing, by and large.
Except the two doctors who started Bioware - they quit because they had a ton of money and were never obsessed with making videogames (indeed, IIRC Bioware not founded explicitly to make videogames but rather to make a piece of medical software, but they all then decided to make a game). One of them I know wanted to primarily run a microbrewery not make games. That's pretty common - one of the founders of CDPR, despite games making bazillions and being their own publisher and so on, quit because he didn't really want to make game, he wanted to run his loss-making restaurant on the millions he'd made making games!
So let's not over-mythologise this.
Also re:
It honestly feels like the name is just a brand now rather than the actual studio that gave us those legendary RPGs.
You could say the same about basically every long-lived company except Bethesda Game Studios.
Like, look at Blizzard - almost no-one who founded Blizzard or came up with the big games or w/e is still there. They've pretty much all quit. But is Blizzard getting worse? No. Blizzard is getting better. WoW improved quite a lot after the sex abuse scandals and so on came out, because it seems a lot of the creeps were kind of holding back a newer generation. Diablo 4 is on a steady trajectory of improvement. It's not for everyone but it's clearly succeeding and undeniably getting better at being the kind of game it is. Overwatch has had some issues over the years, but is pretty much universally agreed to be in a really good place right now, adding a bunch of new character, 6v6 is back, etc. etc.
Whereas look at Bethesda Game Studios - almost everyone at the top of BGS, Todd Howard, Emil Pagliarulo etc. has been there for 20+ years, and indeed we know that's true of senior positions within BGS generally - the same people are at the top working on Elder Scrolls 6 as were on Oblivion and Fallout 3.
But their quality of output has pretty steadily declined (or at best remained static whilst others improve - I'd argue declined though, as they lost the ability to self-edit). Many people see the peak as Morrowind. Some might see it as Skyrim. But FO4 was clearly much worse written, and much full of itself and confused about what kind of game it was than previous ones, and Starfield took most of a decade, but was bland, bloated with generic, forgettable "content" and so wildly outdated in virtually every way that it felt like an HD remake of a game from, well, 15 years ago. And not in a good way!