You can still fill the grid with 3 unique cars though, I mean the real Super GT is like that no?
Besides currently whenever you have free choice, people will just gravitate towards the meta anyway so what's the point of including the old cars in the same group?
The Gr.4 class can still have all those brands. They just need to find suitable cars to replace the FF/4WD, and if it doesn't exist make one. They can literally turn an RCZ into an MR GT3 car, why not turn an Evo into an RWD?
It's an issue because it makes the game look unbalanced and unprofessional. I don't know if you watched the EMEA regional finals, but they have a Gr.1 race at Monza No Chicane. The hybrid LMPs were absolutely destroyed. One guy went from 1st to 6th just because of the car he drove, and these are supposed to be the best drivers in the world.
Gr.3 is the only correctly balanced group in the game. Even Gr.B is dominated by the RCZ because of its meta tyre wear. And don't get me started on the N classes...