If airport capacity in the SE is as constrained as we are told it is and if the economy continues to grow modestly then SEN is highly likely to grow. SEN has advantages to exploit;
1) A new, functional Terminal building with space for easy expansion, if required
2) Direct Rail link into/out of London - this needs to be improved
3) One of the worlds Global cities on its doorstep
4) The arrival of new generation 100-125 seat jets, such as the Embraer E-2 and C-Series which offer costs compatible to A320 models and superior performance from SEN's relatively short runway.
SEN has a long way to go and will likely never compete with LHR/LGW or even STN for airlines, passengers and routes, but given the size of the London market and underused infrastructure which is already in place, I wouldn't bet against SEN.