If it means making you play the game to earn those cars, than I see nothing wrong with that as it makes you feel rewarded for earning them. If PG were to just hand out any brand new car to everyone, you will have those complain that the game is too forgiving, and thus deeming it as "too boring to play now" after driving the new cars for what, five minutes? IMO, I prefer the other method and actually play the game to earn new cars. Besides, PG always will bring back these cars every once and awhile.
That's easier said than done. You see, if it wasn't for the fact that you literally have to grind to earn most of the new cars in GTS, than what you said could've been true. I'm talking about redoing the same events over and over again until you have more than enough credits to buy those cars PD adds into the game via updates, which are well over 1 million credits. You have no other choice but to buy them with real money if you don't want to grind for them, and we all know how bad GTS's economy is that regard. PD should do what Turn 10 does with its DLC cars (and cars in the vanilla game), and that's having them be all available in Arcade Mode. But atlas, PD won't ever that and fans won't demand it. Which in my book, is no better than how PG handles its new cars.
PG will always bring back these cars every once and awhile, whether in certain events on the playlist, the Forzathon shop, or other means to obtain them.