The vast majority of cars are perfectly affordable. Some are downright cheap. Normal playing will get you the ability to buy these cars just fine.
The cars that cost $1 million+ will be affordable by normal playing as well, but obviously only with people who put a bit more time into the game.
Some people are basically using the phrase 'the grind' interchangeably with 'playing the game normally', adding a very negative connotation to what the game is like. Now, if you don't like racing that much, then yes, you may feel like you're just having to grind your way through the game as racing will just be a means to an end. For me, its opposite and buying cars is just a means to race. So it depends on how you look at it, I guess.
As someone who literally hasn't played Forza before except at a demo station I'd be interested to see someone crunch the numbers to see if this is true.
I think with a little bit of time and little math one could deduce whether this Forza has really increased the grind or not and if so by how much.
However the repealing of prize cars and having to earn DLC cars seems pretty blatant to me.