Is 'difficulty shifting the Dashes' actually FlyBe's problem? If they reach the end of their initial 10 year lease, presumably they become someone else's responsibilty. If Flybe try to shed them early, this presumably comes with a financial cost.
I'm also not sure it is fair to describe them as tired. I fly them very frequently, and if you get one of the G-ECxx or G-FLxx series, they are in very good nick from a passenger perspective. OK a shiny new E175 would be better, but they can't swap out 30+ aircraft overnight.