You said that but you want a 1000 car list. There will always be that argument if you have lots of content. You simply can't have GT level of details and a 1000 car list. The same goes with having a detailed environment with Ray Tracing and 60 fps. It won't happen.
What I meant is that one way to ruin the reputation of this game is to cut down on cars under that excuse. That's what happened with Motorsport 5, apparently, giving "quality over quantity" as the main factor why they cut down the car list from Motorsport 4's 650+ to around 200 at launch, considering the time constraints, development on the then-new Xbox One, and that they even had to assist Playground in developing the first Horizon, if I remember closely. Two years really isn't enough to develop a Forza game if you want the most cars and tracks, detailed and polished gameplay, and so on.
Thing is though, every car featured in the series starting from Motorsport 3 are at premium-level quality, given they're fully modeled from the exterior and interior. It just needs some minor polishing to get it to accommodate the latest graphics engine, whether it would be with ray tracing or not. The "standard"-quality cars of Forza, if going by the franchise's history, are those cars that were left out in the first Motorsport, as well as Motorsport 2, that didn't make the jump to Motorsport 3 and onwards.
It's either due to licensing, and possibly even Turn 10's lame rule of having the "latest or fastest" in a car's model family series and leaving all the rest out, why we have lost some good cars in the past.
There is that difference in graphics engines throughout the generations of Forza titles, but either way, that's also the case with PS3 premiums in Gran Turismo - they also need to be rendered under the latest graphics engine with minor polish and quality control rather than having to re-do them from scratch (while for standards, they need to be rebuilt either way).
But then again it would be disastrous if this next Motorsport has a car list smaller than Motorsport 7.