Simple: this MMO industry went free to play between 2010 and 2012.
There are only 3 subscription based MMO's left these days (WoW 8 m, EvE 500k, Rift 250k), all the rest needed to go free to play in order to have "any" players after 6 months.
It started with AD&D in late 2009 and ended with SWTOR in late 2012.
So NO in game "solution" is going to solve anything.
We'll see where the lower sub limit stands for Blizzard to go free to play, but I doubt it will be anywhere before reaching 3 million subscribers.
Besides, it is enough to introduce combo packs to new Blizzard games to pump up subscriptions anytime.