Never had this message in previous GW2 when I overhauled my simple CRJ schedule into a 7-7 MRJ schedule (and we speak about nearly 500 frames, there). I had some real opposition in Alicante, but maybe not enough for the airport to look crowded.
But I can imagine that had I done this in Arlanda(where opposition was harsh, but I arrived only with MRJs, so never had to overhaul my schedules), I mght have met this problem as well.
So I agree with others, we need 2 buttons : "buy all needed slots" and "release all unused slots". Woudln't change anything in the fight against slot-hogging, but would allow for smoother transitions.