In defence of PD and Kaz, none of that has to do with over-promising and under-delivering: that's comparing two different games.
Now, had PD announced GT Sport was going to have more cars than any GT game before it, and
then bumped it down to 150+, you'd have a point.
I don't know about anybody else, but I fully expected a drop in content this generation. It simply wasn't viable for PS2-era assets to be recycled again. We saw it with FM5 — its predecessor had over 600+ cars, so the list was one-third that. That was in 2013 at the launch of the XB1 though, not nearly four years in.
I'm guessing part of the shock for some folks is that they just assumed PS3-era Premium models would make the jump. It wasn't a stretch of the imagination: Yamauchi did state those cars were "PS4 ready" on numerous occasions. And considering the huge quality gap between Standards and Premiums, it certainly seemed like Polyphony would bring the Premos over to PS4 — there's barely a difference there!
Maybe that's where the 500 car count will come from. But then that raises a whole other question: how will the game's esport focus weigh on all those varying models. Can we expect a Group B Schwimmwagen?!