I disagree with this arguement. If they were the same quality as their FM4 counterparts, then sure. But they're possibly remodeled from the ground up, so they make sense as dlc.
If they have a bunch of cars that haven't been completely remodeled since FM1, I wouldn't expect they're going to start now. Though updating old models is fine as long as the original model's proportions are accurate, take the game's Impreza 22B and '99 Viper ACR as examples of how not to re-use assets.
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I don't know what's gotten into Turn 10/PG lately/this time around. DLC has lost any credibility, the car sounds have gotten worse (my Vipers make fart noises at low revs, and when they aren't being flatulent, they sound nothing like a V10), and worst of all, my Alpine GTA still isn't in the game after a WHOLE NOT QUITE A MONTH!!! How dare they?
The route creator is also classically Forza. You can do anything you want, provided it starts in the place that they choose for you, and for what reason? Is it some strange hardware limitation? A lack of resources? Of course not, they're sitting on piles on dosh. It's because they chose having their decorative gantry and barriers at the start location over giving their players freedom, thus almost defeating a purpose of a course creator... And there doesn't seem to be a menu where you can manage the tracks you have made, you only see your custom tracks when you're saving a new one. Though credit where it's due, they did implement it, and that's a start.
Call me crazy (applies tin foil hat), but sometimes it feels like each imperfection in the game is controlled to give incentive for players to get the next game. Though that theory goes out the window when they start making random aspects of the game worse for no apparent reason (sounds).