Yes Stansted does have plenty of potential capacity in the future with a larger terminal. Gatwick manages over 40m pax per year so Stansted can probably find space for 15m more pax beyond the 25m pax who currently use it
But will Stansted also try raising its fees to airports, and will it remain suitable for thinner routes in 5 or 10 years time ? I suspect as STN gets busier, someone at MAG will get a little greedy and want to 'further monetise the customer base' - or in other words raise the charges to make unfeasible in summer anything with less than 140 tickets sold; LTN will probably also structure its airport charges similiarly
As long as SEN management don't get greedy, there will be plenty of space in the market for SEN