I've never seen game developers say that they're simply tired of their own game, and it's not encouraging me to trust this won't happen again
Sure but that’s because a lot of game production is revenue driven and publishers will always prefer milking existing IP, rather than trying something new and risky. „We don’t want to keep doing the same thing over Andover again” is an understandable sentiment, and tends to be a mantra of some of my favourite studios.
On a side note, Larian moving on from BG IP was never in regards of the support of the main game. The initial plan was to make new content (expansion?), so state of the base game would likely not be that much different than it is now. And BG3 is still being worked on, a year after it’s initial release.