On what grounds would it be 'stupid?'
A very valid point is made above about them moving into quieter time slots which will be of benefit to the airport for reasons laid out.
The reason DUB stayed on the boards was because it was extremely lucrative for the airline. Short flight, high demand, higher than average fares as well good on board sales. The reason they have dropped on frequency, as well as at a number of other UK airports is to do with charges at airports and both governments. Ryanair aren't a charity. They find profitable routes and maximise them.
You could argue that they could increase their fares but remember their business model. They've got to remain cheap - their business model and reputation demands it.
FR would be healthy competition for MAN passengers. We live in a democracy with a western capitalist ideology - we're not Beijing.