I wouldn't say that SEN can never reach 2mppa. I think it probably will although wouldn't now like to predict when that might happen.
The big announcement that was expected to be made last month would in fact have resulted in 2mppa very nearly being reached in 2018 and it's still not clear, to me at least, whether it was only a postponement to the plans or a curtailment. Certainly at the moment the planned operation has not been announced anywhere else.
I hope that even the fiercest critics will accept that SEN has been unlucky with the two hoped-for new operations for 2018 not coming to fruition. It seems that the Monarch collapse and resulting slots becoming available plus the Brexit uncertainty contributed greatly to this. Setting up at SEN is obviously seen as more risky than expanding existing LON operations in the current uncertain political and social climate.
It's certainly an undisputed fact that SEN's operational limitations do hinder its growth potential but not to such an extent that 2mppa cannot be achieved in my view.