Originally Posted by
Kirishima
There are too many instances to give you a single one, but in many interviews Nasu talks about how FGO will have an ending that will make you think "it's really over", he wants it to have a proper conclusion. Obviously the game won't shut down, but it's pretty safe to assume he won't give it his full attention anymore.
And he has been talking about that for a while so it's not something he said on a whim. Plus you have the fact that he stopped supervising FGO to focus solely on Tsukihime at the rear end of 2020, so he has no qualms about letting the other writers do their own thing and focus on other projects (it's really clear he wants to get on with Tsukihime and Mahoyo right about now and give FGO the ending he wants, even if we know there's gonna be another part without him but he doesn't seem to care if his job is done)
EDIT: I googled "Nasu FGO ending" and found the two major instances of how he said he has planned on giving it a proper ending on the finale of part 2, from 2019 and 2021 so it shouldn't be too hard to find info on it.
EDIT 2: Furthermore, on asked in a post TsukiRe 4Gamer interview about the future and rest of the remake, he clearly says that his current plans are finishing FGO and then working on the rest of Red Garden, and a culmination of TM phase 2, and start of phase 3. It's pretty clear he doesn't want to be stuck to FGO.