I think if Ryanair or Easyjet could make a successful thing out of Manston other airlines might follow.
The Davies Commission makes the point in relation to Gatwick (but it applies equally, if not more, to somewhere like Manston) that the probability of any of the alliances moving there from LHR is extremely remote. Add to that the non-allied carriers who depend significantly on interline traffic from the big 3, and that leaves very few airlines indeed who would be likely to volunteer to leave Heathrow for an airport in the sticks.