Metro Man, is that O/D traffic or is it transfer?
In the first case, this number is not unlikely to shrink with less EU citizens taking up residency in the UK and travelling between their original country and their new home.
In the second case, this is likely to shrink as well as long as there is no open skies agreement. This will benefit the continental European airlines not insignificantly, so expect pressure from those on the EU to mind their position when negotiating the impending divorce.