Indeed EZY did shift capacity to SEN... and as I remember arguing quite strongly at the time, in the medium to longer-term somebody would see STN as an opportunity. Needless to say that was ridiculed for STN being in the wrong place and that nobody dare go up against FR blah blah... Though Jet2 turned out to be that 'somebody' and it can be quite confidently said it's launch (since that's all any of us can go by so far) has been a resounding success.
Nonetheless STN is still a good earner for EZY and I wouldn't even argue that STN is any less yielding for them than any other London airport. Even if so, much of that would come down to intricate details of the commercial deals and growth incentives they have with those airports. The truth is, LGW and LTN is where their appetite for growth is at. In 10 years time however, they may get a poorer deal from those airports or a key change in the market could mean that appetite then moves to STN.
What I'm getting at... is that it's not a simple case of one airport being less profitable than another. It overwhelmingly depends on what the airline is actually willing to do at that airport and, in part, the relationship an airline has with a particular airport.