I rebuilt the engine in my 1961 truck 3 years ago, the bores were worn enough it was noticeably down on power. I can't easily track MPG in it for a precise number (no working odometer), but the mileage increase was significant enough to notice a difference at the gas pump, I'd estimate a 4-5mpg improvement. This would be an extreme case though, I really don't know how that engine even still had enough compression to start. The ring end-gap was slightly over 1/8" (0.128"), spec is 0.016", so on the extreme end of engine wear.

To get back to EVs though, I'm not really sure they will last any longer than current ICE cars. Engine reliability has gotten good enough that a worn engine normally isn't the reason a car gets taken off the road. IME the main killer is either body rust or just too many small parts being worn out to where it isn't cost effective to keep repairing. Suspension parts will wear faster on an EV, since they're heavier than equivalent size ICE cars. I've driven a lot of mechanic specials over the years, and of the 7 cars I've sold to salvage yards only 2 were due to engine issues, the rest were either body rust making them unsafe or just too many things wearing out.