Herne Bay exaggerates the difficulty of getting to Heathrow or Stansted from Kent. A couple of weeks ago I tried to get up to Maidstone from Thanet. The police had shut the M2 following an accident. I sat in the traffic for an hour or so and then tried to get off onto the A2. Everyone else had tried the same thing and the A2 was completely jammed. Eventually, after three hours, I gave up, turned around and went home. It is highly misleading to suggest that East Kent is easily accessible. There's basically one road in and one road out. If that road gets blocked you're stuffed.
If it were economically viable to open routes for the South-East it's far better to do it from Gatwick than Manston, because it's far more central and has a bigger catchment area. What you seem to be suggesting is that it's economically viable to operate routes from Manston that are not considered to be economically viable from Gatwick. Poppycock.