We've been using STN as our main airport for years (admittedly nearly always leisure) and have preferred it to all the other London airports (apart from LCY) for ease of use. For anyone living in NE and E London it is definitely the easiest airport to access (ignoring LCY) although this may change with Crossrail 1. It is looking a bit down-at-heel at the moment but I put that down to the whole sale palaver.
The London / STN / Cambridge artery has been targeted as a major growth area, so there is no reason why it shouldn't continue to flourish. It is quite a long way from the rest of the UK though and certainly the catchment area for LGW / LHR and possibly even LTN is more affluent for the time being.
It's highly unlikely that Crossrail 1 will be modified to include a spur to STN. This wouldn't make it more accessible anyway because there isn't spare capacity for faster trains. This will have to be built and indeed is planned (not budgeted) for some time in the medium term. Crossrail 2, which will be essential if HS2 is built to ease access to Euston, would probably include a link to STN. But I doubt that this will increase access speed much either, without the aforementioned infrastructure improvements.
From a personal point of view, the worst thing about rail access to STN is the price, not the speed. I could easily catch the train every time I go there but don't because it's too expensive. Coaches are half the price and only marginally slower. Even if the train was only 30 mins I wouldn't catch it. Not an issue for most business users but I suspect that most business users live further West and North, though that's changing as the East becomes more affluent.