Have a PS4 Pro (that I disappointingly picked up for GTS, oof). I won't be upgrading to a PS5.
I love Gran Turismo, I've driven 80,000kms between GT5 and GT6, I haven't got my monies worth on my PS4 Pro and $750AUD (if I'm lucky and quick) + $110 for the PS5 version, nu-uh, that's pretty silly.
If I had a PS5 for multiple games, no question, the PS5 version will be superior, however in my situation what I'm really going to miss will be.
120fps (has this even been confirmed? I think so)
Loading times
Smoother 4K performance
Controller upgrades which I wouldn't care about if using a wheel
That's not $770 worth to me (PS4 version is $90 vs PS5 at $110, $155 for 25th Anniversary edition $45 of DLC!? c'mon).
Besides, PS5s won't be expensive forever, given how early into the lifecycle GT7 is launching, I will probably wait for GT8 and a PS5 slim/pro.