Well the math is rather easy, if a player averages a million credits an hour grinding, and we assume the gold all the Track Experiences and Missions (which is not going to be true for all), and they play for an hour a day on average, it will take them a solid year of grinding to get all the cars in the game (and that's assuming no changes to Legendary prices and doesn't count new cars being added).
A year of running the same event in the same car, over and over and over and over again.
That's quite clearly describing a fundamentally broken game mechanic, one that PD is happy to see broken (because MTX), as clearly demonstrated by the fact that they nerf every high credit grind event as soon as they can after its unearthed by players.
To be blunt PD have played a slight-of-hand that a worrying number of people are quite happy to accept. The game economy at launch was broken, PD then made it worse, before rolling it back to a similar level to the launch one, PD didn't fix the in-game economy, they simply moved it from absurdly broken back to just plain broken. Oh, and no, locking high earning races in missions to make the pay-outs a one-off wasn't a fix either, rather further evidence that this is how PD want the game economy.