And yet, to construct it, they built up from the past game technology rather than "start from scratch". Thus GT 2000, which much like GT HD Concept was a mix of the old game and some new technology.
If it happens, a tandem PS5/PS4 release would be the first time they've actually released a GT product (or are least are entertaining the plans; GT7 for PS4 has not yet been announced or listed, it may still be in evaluation while the PS5 product progresses and/or may be a post-release version if they do it) that cuts down to the past gen as well. Usually the Gran Turismo engine has a lot of custom work that isn't friendly for scalability (one of the reasons for the prolonged development timeline for GT PSP, though "perfectionism" ultimately made "GT4 Mobile" not happen in 2005 or 2006 as planned; tech might also have been a factor in the sad lack of a GT release on Vita.) With the way games tend to be built these days, however, the engine would likely be more flexible just by common practice alone (even if an engine is in-house, I don't know if there is a "to the metal" with something like PS5?) From what we've seen of next-gen products so far (and from what we're seeing of projects that are still scoped into 2022 and beyond,) there's not a lot of technology or project scope yet that would set up a dividing line where cross-gen and port-downs are not feasible.
I suppose we will see just how different these next gen and last gen versions really are. But typically and by that I mean pretty much in all cases, cross gen games are identical to last gen versions save for some visual enhancements like ray tracing, higher resolutions and higher framerates. Devs just dont bother implementing features that will basically double their work when QA'ng core gameplay features. I highly doubt they will have a different physics engine for next gen. I bet the cars will control identically across both platforms. They might cut various graphical features like smaller crowds, 2d trees etc, but the core gameplay and that means stuff like destruction and other A.I related stuff will be the same.
I was looking at some of the Horizon gifs today for folks calling the first trailer CG and while they look identical to what they showed last year in the cutscene only trailer, they dont even come close to what Avatar is doing. They even hinted at flying being in the game, but its obvious that the visuals just dont look as good when we look at them from the sky. The LODs take a massive hit and the detail just isnt on par with what we see on the ground.
And thats just due to the game being held back by last gen hardware. If the flying is in, it's in both versions. If it's in both versions, you are not going to see Aloy fly faster on the PS5 version simply because that would require them to readjust entire gameplay systems around the higher speeds. It's why we didnt see Miles swing faster on the PS5. The game is designed around the PS4 hardware limitations.
Even if Ubisoft ends up downgrading the shit out of the gif below, it will still look a gen ahead of Horizon because they are using the SSD to stream in assets while flying at high speeds. Their words. This is something even GT couldve used to really push the graphics.
P.S You know whats funny? A couple of days before Avatar was surprisingly announced, I posted some gifs from the movie of them flying at high speeds and riding on the cheetah at high speeds in a discussion about Horizon being held back. People laughed and mocked me for thinking I am expecting way too much. Well, we now have an Avatar game producing these insane visuals doing exactly what I wanted. I wish I could remember what thread that was so i can bump it. ;p
People just dont realize just how powerful these next gen systems are. Primitive shaders are a game changer. SSDs are a game changer. The PS5 and Xbox I/Os and CPUs should let devs go insane. The Avatar visuals are pretty much what we should expect from AAA studios like Naughty Dog and Rockstar. I expect some late gen games to look even better.