As much as I want to keep seeing cars that have been in the GT series for a long time continue to appear in the next GT game, I don't think that's a realistic thing to expect anymore so I try not to get too attached to them. Even Forza dropped cars that appeared in their first games. FM1 and FM2 had a lot of cool modified variations of Japanese cars like the 1,000 HP VeliSide Supra, but for whatever reason, they had to let them go at some point.
Imagine seeing FM1 and FM2 quality cars in a current Forza game. Turn 10 would receive a lot of negativity for it and it would hurt their reputation, just like with PD. For the good of the GT series, I think they need to just leave those cars behind if they're not going to update them.