I don't think Jet2 will ever up sticks and leave LBA, they have invested far too much into the place, it would be pretty crazy to leave the place when their HQ is there BUT I know exactly what you mean.
As for the deal that Ryanair have got, its nothing fantastic at all, and O'Leary has been public that they did not get the deal they were looking for. Jet2 have been treated very very well at LBA in certain aspects and I really hope this continues. However it was imperative that the new terminal was in place before Ryanair move in, and this is not going to happen. For passenger figures to grow by the rate the LBA management team want, they needed to get Ryanair in, but not for Ryanair to duplicate routes already served very well at the airport.
I believe that if Ryanair opened a base at LBA serving routes not already served, and routes that the airport has made public they are desperatly looking for - Frankfurt, Bremen, Madrid, Stockholm .. all Ryanair bases. This way they could have kept Jet2 happy with the wealth of routes they perate from LBA, and grow passenger numbers. I know Mr Meeson is very annoyed at the route duplication, but he is confident that Jet2 will win on many of the routes.
I'm not sure if it has been posted yet but Jet2 have also dropped PSA from LBA as well as KRK. I fear VCE may be next, but has PM has said, the city routes are performing rather poorly and his business model has changed to the core sun leisure routes, this has been backed up by Jet2 winning several large contracts with Thomas Cook recently, with Jet2 operating charters for TCX to HER, FAO, SSH, TFS and AGP next summer from LBA, allowing the TCX based unit to expand into new destinations. Jet2 also winning contracts over Viking Airlines at MAN with their new KGS, CFU (etc) routes. A very good way to minimise risk on new routes.