Not having access to Blizzard's exit surveys, we don't know why people are leaving it. But considering that it's nearly nine years old, it's not surprising that the ridiculously high numbers of subscribers are starting to fall down towards proportions more typical of MMOs (a couple million players tops--Prior to WoW, the biggest MMO had barely half a million players, and of the hundreds of MMOs out there, numbers of under a million active players are very common). League of Legends isn't an MMORPG like WoW or ESO either. It's a MOBA. Different style of game for a different target audience.
As a side note, the various so-called "WoW killers" all suffered from the same problem--They were basically WoW with a different setting. Most post-WoW MMOs fall into that category.
Final Fantasy 14 relaunches next week. They're still going with a subscription only payment model. One of the advantages to a company like Square-Enix or Zenimax making the game with their own money--They can take the long view on making it back and go with whatever payment model that they feel is best for the game, rather than having to make a cash grab according to what's best for the investors wallets.