That's indeed the problem, if somebody doesn't keep ordering something like the Metroliner or E110, you end up dead, or dependent on the used market for what you can get. I'm split right now between the E110 and E120 in my game, with a 100 airplane fleet now (80% of it is E110). I don't make a ton of money, but nobody else messes around with these routes that have between 10 and 30 demand for the most part, so it's easy to get routes going. I'm going to try to stay with an all Embraer fleet and expand up to the ERJ145.
What would make running an airline like this make sense is if there was some sort of fee for departure model set up, like the real regionals have in the United States. A large airline could contract it's regional feed out to another airline and pay them to do it.