Look at Diablo 3 and tell me that payment methods don't matter.
You can't just say "well if you don't like it, don't use it" when the game is designed from the ground up around it. I'm not necessarily saying this is the case with Forza, but that IS what some reviewers and players are saying.
If cars are no harder to get than in Forza 4 or previously, then nothing has changed. However, the fear as a player is a developer designing the game in a way to incentivize microtransaction purchases. If they make the game grindier to try to get people to buy cars, then that is unacceptable to me as a player. That is hamstringing your game for most users just to make extra $$$ off a few.
Again, I'm not saying that is true for Forza 5. I haven't played it. But some people ARE saying it is true. People in this thread who have the game have commented or can comment on the relative grindyness compared to Forza 4 and below.
Just look at games like D3 with it's AH (gold, not just real money) and PvZ2 with its extremely grindy nature. You can't simply say 'if you don't like that part of the game, don't use it' because the entire game is designed around it.