Where cross-gen will hold it back is the amount of cars on track. It being cross-gen rules out multi-class endurance events with 200 cars on track. The yearly Nurburgring 24 hour race won't be possible if it has to keep parity with the ps4 version. The PS5 should be able to handle AI for 200 cars, ps4 likely can't. GT Sport already had a bit of trouble with 20 car standing race starts. (Fine with rolling starts, but too many cars too close together had fps dips, on ps4 pro)
I am happy the game is cross generation, first because more of us gamers have the chance to play it, second, because it gets more sales, which means more fund for the developer to developer updates and more games in the future. Unfortunately there's a shortage of consoles, if ps5s were widely available then my opinion would change. But if the developer doesn't get enough sales it could mean the end of the GT series and no one wants that.
24 hours race is a nice option, but honestly I struggle to have 3 hours straight to game nowadays, let alone 24, if you game for 24 hours straight then you have a bigger problem than GT being cross gen.
Dynamic weather and 24 cars on track is very possible, it was done on forza on xbox one, it could have been done on a base ps4.