And there is a very clear reason for that, if they were to have only premium cars then half of the grid variety would be gone, and the game would suffer dearly for it.
I disagree that a game needs a thousand cars to have a decent variety on the grid. A well selected roster of cars in sensible classes is capable of doing everything that a monstrous list of cars is, duplicate bollocks notwithstanding.
Even with the 1200 cars it has, GT6 sometimes struggles to fill out fields with equivalent performance cars. It's not about numbers, it's about sensible selection.
I'm not saying that I want grids full of the same car either, but to suggest that it's an inevitable result of reducing the car list is a strawman.
GT6 already suffers from some pretty weird choices of what to premiumise. I think with no standards propping them up, they might have to sit down and plan out exactly what cars they want in the game, how they would fit together in fields on the grid, and how they would cover the range of both recently released new cars that people might be buying and old classics.
GT6 solves the problem (sort of) through sheer volume, and it's not the only solution. The GT car list appears to be designed by someone saying "that's a cool car, let's put it in!", instead of "we need another car to fill out the field of '90s sports saloons, what's the best thing we don't have yet?"