What? Of course there's a way around it: have compelling content that people actually want to play.
There's always going to be decline as time goes on, but the sharp crater of WoD was in no way inevitable, especially after the initial launch surge. If WoD was a better expansion overall (with a better patch cycle) the current numbers would probably have stabilized around 7-8mil, similar to the path that subs took during Mists.
Subs will decline whatever they do, the game is old and we're bored faster than before. They never boast about
new subscribers because there aren't that many. It's only about people returning when you listen to them.
Also, MoP was pretty heavy in patches and new stuff until the final drought and subs declined anyway. Worse, people kept complaining content was coming
too fast. Blizzard took notice, and the various graphics mmochampion often publish show that the majority of players don't finish content anyway.
So, in a business sense, what do you do then? What they're doing. Faster xpacs, lighter on content (because Blizzard is notoriously slow and mismanaged, and they try to allow ressources to other projects, faster to develop) and you try to minimize losses by instead counting on the annual xpac price to counterbalance sub drops.
They could put a new raid each month people would leave anyway, and some would return later. They've said multiple times patches don't really make people come back, xpacs do.
I wouldn't expect bigger expansions from now on honestly. They can't do it in a reasonable timeframe and it would be a waste of time and ressources. They will try to not do worse than WoD though, obviously.
They could have handled that better and lose less subscribers, of course, we all agree on that. But I don't believe for one second that WoD would have kept bigger numbers than pandaria whatever had happened.