I'm kind of in the middle. While I do try to be reasonably realistic, I like some of the fantasy paint schemes whether it's railroads that never actually owned that particular engine or train car, stuff painted for freelance railroads, or the old promotional paint schemes like the ones you saw in trainsets back in the day like Tootsie Roll boxcars and Coca Cola tanker cars. I also have some cars and engines that are foobies. Granted I do try to keep everything in the 1980 to present era and stuff that would appear in western Pennsylvania as to me I just cringe at the idea of a steam locomotive pulling a double stack intermodal train, GEVOs pulling wooden reefers and boxcars with roofwalks or the Super Chief passing through what is supposed to be Pittsburgh. Also, because it helps cut down on a lot of those "gotta have it because it looks cool" impulse buys.
That said if the layout owner is cool with people running modern stuff on his transition era layout but they don't like it that's their problem. They can start their own club, host their own sessions, or go elsewhere.