GT(7) is a console seller. I'd still be on PS4 if it was not for GT7.
Same with me, but twice bitten with GT Sport and PS4, and now a PS5 and GT7, and never again. No PS6 for me, even if a GT8 were to only be on that console.
If GT8 were to also come out on the PS5, as GT7 has also come out on PS4, I'd be very wary of getting it unless there was evidence of it being a significant improvement over GT7. And I can't see that happening. They had many years to get GT7 right, and for me they majorly fumbled it. But they have not really addressed a lot of the criticisms to any great degree imho, like they don't really know what a GT game should be. Or maybe they no longer want to make the type of game that a lot of people wanted, or were expecting.
Either way, I can't see the next game going back to anything near what we had before, when they are choosing not to change GT7 to any great degree this far after release. Spec II did little for me sadly, too little for me, very late.
As always your view may differ, and that is fine.