What's funny is I think all of the major game studios that I know of essentially lock game engine per project once they're coding along. It's just expected that you stick with it for the rest of the project. Maybe 1 upgrade mid project. I had heard bungie used to, in the halo days, lock the engine 8-11 months out and the core tech team started working on the next game. The AAA studio I worked for essentially did that 3-6 months out.

But those were 2-3 year game cycles. Maintaining a project would be less fun. But it'd be like choosing to do a rails or react upgrade.