Don't think PvP is much of a reason. If you compare 5.2 to the start of S11 balance is actually a lot better. But people quit if their class gets nerfed (warriors 5.2, priests & rogues in 5.3) because most of PvP players like to play a class only if it's at least a bit too strong. Means better balance can actually hurt the PvP subscription numbers in short terms.
Overall this expansion is awesome and in my opinion the best expansion so far. There is a ton of stuff I would like to do but can't due to lack of time (Challenge Modes, Brawler's Guild, legendary quest).
People quit all the time for many reasons.
The issue however is (talking about PvP here):
The start into the game is horrible. If you are 90 and are happy to start with pvp you have no gear, have to endure random groups with people botting, pretty much no team play. No achievements and you only get RBG groups with pretty much zero coordination, no one to explain you anything and a kick after looking 2 hours for an invite without comment. The guilds are not social, no helping/explaining, no one responding in the guild chat. Means if you don't know someone that is willing to explain and carry you - you are pretty much screwed. And the only source of help you can easily find is the boosting ads selling you access to RBG groups by selling achievements. If the intro into PvP isn't eased and people get a chance to actually learn and improve PvP will not cause new or lasting subscriptions.