And all of the reasons why GT7 was an exception will almost certainly be doubly true when GT8 comes out.
Ya, no, not even close.
PS5 was released in 2020, during a global pandemic. That pandemic caused supply chain issues (and various supply chain issues persist or have resurfaced) so the number of PS5's in the market was subdued, leaving PS4 as a viable platform.
That pandemic also weighed on the development of GT7, likely delaying it significantly because it's time between iterations is the second largest gap in the title's history (4+ years) without the massive technological leap between GT4 and GT5 (about 5.5 years).
So, unless there is another global pandemic "all the reasons why GT7 was an exception" do not exist.
PS5 is out now. There is no release date for PS6. The franchise's 30th anniversary is next year and we are already 4.5 years into this version's life cycle. If GT8 releases for next Autumn/Christmas, it will be on par with GT5 for the longest gap between iterations.
These the last few updates indicate they are still hard at work on the underlying tech and assets of this generation. Spec IV coming. So, who is hard on a next generation GT8? No one, because there is likely no hardware to work on, and they'll need that hardware for a significant amount of time to get the game working and looking like it's worth the upgrade.
On the flip side, all the updates GT7 has gotten (the Hypercars and all the new models) would also be part of GT8, as has been the case in the past. So, you could say that they significant GT7 updates (and they are significant for a 4.5 year old game) are, indeed, the development of GT8 as well.
I don't think Sony knows what PS6 is going to be. I feel like they'll have something akin to DLSS so that next gen games look next gen, and all the current gen games get an instant boost if you play them on PS6. In any case, I think delaying GT8 until there is a PS6 is an insane idea from a business sense. You have a rabid audience that will stampede to buy GT8 and you'll finally get the stragglers off PS4.
Lumping GT8 and PS6 together makes no sense.