I DO know what GT is.
But times change, tastes change and markets change. Hell, your own userbase can change.
As BoilersFan23 mentioned, GT3 supposedly had less content than 2 and yet sold amazingly. History has shown us this gen more than any other that streamlining your product usually results in better sales.
Who is talking about cutting existing content?
I'm going to assume, given PD's MO, that they're planning let's say 200 or so premium cars for GT6, and they'lll carry over whatever is in 5 to use in 6.
Instead of 200, make it 100 and choose the cars more carefully. Maybe PD should invest some time and money in research into which cars are wanted?
Use the website and do surveys. Get information from the existing userbase, find out what they want. They might find out that no one is touching Skylines anymore, so all of a suden they can stop creating 87 of them in every future version of Gran Turismo.
This is all hypothetical of course but you see what I'm getting at yeah?