For Manston-Amsterdam - as for other short routes - the problem is that there is almost no cruise phase, and what there is is at a lower altitude than normal, so less fuel efficient. On top of that the MSE-AMS route crosses major east-west inbound routes into the London airports and any KLM flights would have to negotiate entry into that busy bit of controlled airspace. Not of course impossible but likely to complicate flight planning and delay/interrupt the climb.
And Thanet has a sufficient catchment area to fill 160 seats in each direction to/from AMS daily?
It would almost certainly have to be 240 seats a day each way since KLM doesn't do less than three rotations a day to any UK destination.
But it would be a major coup for Infratil (or new owners) if they can achieve it.
NS