STN v SEN
It's not necesarily and only about catchment areas, SEN has a unique selling point as a small airport that's easy to get through with minimal hassle, a bit like LCY. That can be a big attraction for many. As mentioned above, SEN will not go above 2,000,000 pax/year, so will always have this advantage over STN.
Suspect that if NHT was up and running as a small regional airport, there would be those who would prefer it to LHR (even if it was further) for exactly the same reasons.
As for catchment areas, those of STN, LTN, LGW and LHR all overlap. If U2 lose East Anglia based pax by reducing their STN operations, it could be repaced by pax from the south coast (LGW) or from parts of the Midlands (LTN).
Many pax could probably go to all three of U2's large "London" bases, so if one is run down, it is not the end of the world. Doubtless U2 have sound commercial reasons for its decision.