There are three possible options in the career, if you finish top 3, you get promoted to the next difficulty, if you finish in the bottom 3 (or retire) you get demoted, if you finish elsewhere you stay at the same difficulty, I don't see this as an issue, because what's the point of pushing the difficulty if you can't finish top 3?
The only rewards for higher difficulty is more winnings and longer rallies, so there is no real issue with staying in a difficulty that suits your own skill level.
Not sure how else they could have handled the career, apart from manually selected difficulty level, but that wouldn't have really changed much.
Sim racers are just like this, it's not GT or Forza, people leveled similar complaints at PCARS career, I don't have an issue with either , and I prefer a more open natured career , where i can just enjoy the racing, rather than the restrictive, grind heavy careers of GT or Forza, where you MUST finish in the top three to progress.
You don't Have to finish anywhere in Dirt to progress, it'll just adjust the difficulty if you don't perform well.