Demonstrably false statement. Not to mention that they openly stated that the changes they made between GT5 and GT6 was to allow the engine to grow with the platform. Asset wise, this is also visibly false. From GT5 onward, yes. From GTS onward, absolutely not.
...like almost all (barring the new cars and tracks, obviously)
That's funny, we have no idea if a "PS6" will even exist or what the process of creation will be by that time, so saying that the models will be "good enough" is WAY reaching. Still, the GT5 models have been good enough for GT7 (both PS4 and PS5 versions) so maybe they'll keep going.
Keep in mind, streaming is growing. "PS6" might be a completely different platform focused on a different medium of delivery, that might actually require a downgrade in graphics....we do not know. Until it's in the hands of actual developers, we don't know anything of the requirements.
"optimizing" Gt7 for PS4? You have it backwards. It's not like this game was built for PS5 and they ported it down to PS4.
And ray traced lighting is a coding feature, not an assets feature. If they really go hog wild with ray tracing, they could literally completely rerelease GT5, with NO changes at all, GT5 will look spectacular and it won't cost them any more than the coding time of a couple of people.
Which brings it back to the original question, will they make another. Since the DO NOT NEED to reinvest in all that content, the likelihood is high. If what you believe to be true, was in fact true, they would never invest in another version of this game.