easyJet has been increasing its BRS size nearly every year - should have been 18 based aircraft (10 320s, 6 319s, 2 321s) in 2020 had the pandemic not intervened.
Ryanair is still a substantail presence at BRS. In 2012 it did have a fallout with the airport reputedly over parking charges and reduced its base size from five aircraft to two at that time, but it maintained all its services by using aircraft from other bases. Since then the base has grown again but Ryanair also still uses aircraft from other bases to augment its BRS network. TUI is also a notable presence at BRS - pre-pandemic it should have had six narrow bodies and a B787 based for summer 2020. Thomas Cook had three based aircraft at BRS and has never been fully replaced, so Jet2 probably thought there were some gaps to plug.
Just listened to an interview with Jet2 CEO Steve Heapy who made the point that they are aware of the competition at BRS. It was not a snap decision to go there but one they have been considering for a while.