NorthSouth
The fact that Ryanair's 738s will not be able to operate effectively from SEN is one of its best selling points as far as other airlines are concerned I'm sure you will agree, so that's no disadvantage.
It's rather disingenuous to suggest that SEN's declared distances will be to any significant amount less than those at SOU and its TORA and ASDA will in fact be greater.
I'm sure Flybe will be showing interest in SEN following its recent EMB 175 order and that type will be well suited to SEN's runway and I wouldn't rule our Easyjet operating the A319 either.
I don't think anyone is suggesting that SEN is about to become the new Luton, after all their aim is only 2 million ppa, but I think you're adopting rather too much a 'glass half empty' attitude to its prospects if I may say so.