That's a fair point, but there are other issues at play here. I would suspect the easy jet route to EDI was dropped not because the suited brigade don't want to go to SEN, but a single daily flight on a route which should easily be doable for the suits as a day trip doesn't work, and I suspect it is the business traffic that drives these routes and keeps them going so a single daily was sort of always a bit of a risk.
it seems to me the SEN is not in the near future going to be seen as a major London airport in the way Gatwick is, but this is as much to do with the limited capacity of the airport. Let's face it, 99% of the country have never used London City and probably never will, but it's still a London airport it just serves its own niche.
In addition, SEN is finding and will continue to develop it's own niche. Not everyone wants to go central London and for the east parts of the city SEN is a great alternative.
For routes like EDI however it's a frequency / capacity issue. Now I would suspect a regional with a 50-70 seat aircraft operating 2-3 daily returns would work beautifully for something like EDI - SEN.
And so what if it's on the coast? Gatwick is half way to brighton - it's just an awareness issue and the more people that use SEN the more it will be understood.
I'm not into one airport vs another, it's a fruitless exercise I am just saying SEN is just as good an alternative for many parts of London and it has a role to play … and yes teething problems aside it's got off to a good start