I personally have no issue with the legendary cars costing as much as they do, or with the premise that it would take much longer to earn them than the 'fastest cars' in the game. However, I should have the capacity to earn the credits needed to purchase them in a reasonable timeframe, playing the game however I choose.
When your options are (a) play the game for fun (however you prefer, engaging with various different in-game experiences/races), but make the prospect of affording the legendary cars in a reasonable timeframe much more remote; (b) race one of the 'optimal' credit earning races over and over again for tens of hours (to the point of absolute tedium), or; (c) purchase MTX as means of relieving the need to grind, allowing you to play the game for 'fun' (at a much greater cost than the initial purchase of the game), then something is very wrong.