I think what a lot of you get hung up on is the idea of Nasu stepping down being the same as FGO being shut down. Sure, that interview linked above does mention closing service, later interviews have all leaned more towards Nasu himself stepping down, and these aren't necessarily the same thing. Nasu can end the "main story" of FGO without end of service, and just leave whatever side story-esque thing that comes after it up to other writers. He doesn't have to choose between story integrity and making money, and nobody should honestly expect him to.