1. GT6 does not have 1000+ unique cars by any means. This has been discussed to death already.
https://www.gtplanet.net/forum/threads/car-list-demographic.307635/
2. To put it in perspective, Turn10 has done those interiors with that same amount of "unique" cars (those that count because the rest is just copypasting) therefore it is not impossible to PD by any means,
mind you them with a lesser budget and less time.
3. Finally, you are thinking there's only one solution to that problem, which would be "adding interiors" to those cars. In reality what should be done is the opposite, what every single company does besides PD, which is removing cars and assets that aren't up to par, those from over 10 years ago. PD has not done that for the same reason this game has so many duplicates: car count, marketing.
Imagine if Capcom were to use the street fighter 3 sprites and earlier in ultra street fighter 4, like it was a MUGEN game with hundreds of fighters just to claim it has so many characters, even though there's 100 ryu variants, 90% look beyond awful and the game is laughably inconsistent. That mindblowing and unprofessional job is what PD does with the GT series and somehow people don't complain about that as much as they should. No quality control whatsoever, mind you which goes beyond graphics for instance in handling simulation (comparing a standard GT car to iRacing or to real life is unbelievable).
PD has to be the only example in AAA videogames that uses assets from over 10 years ago in their main products, then somehow for that very reason some people defend them on the grounds of that there's too many assets to improve, furthermore say that 2001 ryu sprite shouldn't be removed. That's absurd, all four of them.