The last couple are MoP and WoD, Cata is not it. The drop in Cata was due to HARD dungeons issue, dragon soul drought and SWTOR release. Flying was never a reason for people leaving. NEVER. Hell I left Cata for SWTOR and then returned at the end of Cata. You have to look at the whole picture here. There are external reasons for sub drops too. MoP failed to keep those who left during Cata and returned for MoP. WoD lost even more and this time around there were no external reasons. Except for FF and GW introducing flight mechanics.
Flying is the core reason. Because no flying ruins the rest of the game. So even people who think they left because they had nothing to do - they left because of no flying. Because of how it affects the things you can do.
I know that it isn't the case, otherwise Blizzard would resume releasing sub numbers.