The fear is that Polyphony, a developer that has historically struggled to get it's games running well on a single console, is now going to try and release a game on essentially three (PS4, PS4 Pro and PS5). Polyphony specifically is the scary bit of that combination.
And more specifically:
Everyone knows why FH5 is cross-gen. The next iteration of Forzatech is coming with FM8, and with that, a move to next gen exclusively. The next Horizon, as such, will be a next gen only title. It makes sense to put FH5 on as many platforms as possible considering when it released, and the bull rush to get next gen consoles leading more people to stick with their older consoles out of necessity. You know that the next title in the series is going to come at a time when next gen adoption is much higher, and can push through changes that are needed for both the series, and push the envelope graphically.
GT7, on the other hand, is coming from a developer who has, for the most part, valued graphical fidelity to an almost hilariously one sided degree. They are essentially playing the early game when there is no guarantee there will even be a second GT title on PS5, considering Polyphony were very much alright with sitting on GTS exclusively for the last generation. So, you are basically taking a game that absolutely should
be next gen only, and more or less putting a ceiling on what one should expect graphically, especially on next gen systems, with no guarantee as to when the next game in the series will be that one can expect it to be next gen exclusive.
There's a reason why people are saying GT7 is going to be 'compromised', and that's simply because Polyphony are running antithetical to their usual development ethos, and that the potential short term gains are simply not enough to offset the fact that they are dragging along old, antiquated internal technology in both the PS4 and PS4 Pro for no real reason, and no real sign that the next game in the series is coming.