The issue is that Ryanair have no-where else to go. Yes, they can cut a few movements from STN, but the only economic hub that they have in SE England is STN, and there is nowhere else with sufficient capacity where they can move to.
Like it or not, Ryanair and STN are stuck with each other - but at least STN/MAG have the possibility of attracting more customers, whereas Ryanair can posture, but cannot easily move, say 50% of their flights to another airport.