Exeter and Southampton are so closely linked to the strategy and fortunes of one airline (flybe) that makes them difficult to read. Both benefit from relatively affluent catchment areas, so any decline will probably be modest, unless something happens to FlyBe (which does appear fairly robust at present).
Plymouth has already lost the Air Wales routes, and is intricately involved with Air Southwest - including common ownership. If ASW prosper, so will PLH, but the opposite applies too.
As for NQY, I think some recent successes have been symptomatic of the froth at the top of the boom, and will reverse. BA have already pulled out, but I see Jet2 are planning flights. Not sure whether these will work.
