Well, we saw lot of gameplay footage so far. And the times are different now, because the PS5 hardware (and Xbox Series X) have strong CPU (compared to PS3 and XB360). Especially the PS3 was extremely hard to program too. Nowadays almost all consoles are PC like, meaning its not that difficult anymore. In example, just because some games on the Nintendo 64 didn't play well, doesn't mean later consoles from Nintendo had the same issues. Throughout the lifetime of this console generation we had the most 60 fps games ever. Given Rockstar does not put multiple games in short amount of time like in PS3 days, we can assume there is lot of optimization going on. I mean we saw the gameplay, at least to some degree.

In short, I do expect GTA 6 being playable at least at 30 fps (maybe with drops here and there). But this generation was pretty good so far. You forgot that when GTA 5 came out, that most games were bad performance on those consoles. So it was not something like GTA 5 being especially bad.