I don't buy the suggestion that the popularity of domestic flying gives any wider indications as to the viability of an airfield at all. For most UK airports they make up only a very small part of the pie, and especially so for those (like DSA) whose bread and butter of passenger ops (itself one part of the business) is Thomson sun routes and ad hoc charters. Besides, given the planned reduction in flying later this year, from a commercial point of view it would seem that the Flybe operation isn't particularly self-sufficient without DSA subsidy.
The impact of Brexit on Wizz will be most interesting to observe. We know that EU nationals living here already will have the right to continue to do so, and there's no obvious reason why Brexit will have any substantial effect on their marginal propensity to fly. So I wonder whether we will merely see a plateau in the growth of Wizz's UK ops, rather than a more notable diminution.