HH6702,
Ryanair have 3 based at LBA, and Jet2 have added aircraft at LBA since basing.
Ryanair have 7/8 based at MAN, and again, have added aircraft since the Ryanair base started. In fact, MAN is now Jet2's biggest base.
So, Ryanair basing is not a prelude to Jet2 stagnating/shrinking.