This type of game is created as the Tower of Babel.
 
1) It should always contain a lot of content unexplored by the player.
Therefore, the game has constant updates with new levels and locations. With the new boundaries of player progression.

2) At the same time the game must regularly update the old locations, so as not to get bored.

The game should give an unlimited number of places for research, limitless space for the progression of the character and an infinite amount of content.

While she is giving it she is interesting to the players. But if the whole game consisted of a small location with a small amount of content, the game would be boring.

Imagine that the whole game would consist only of the Pandaren Island. Then the game bored the players in a couple of days and the players would go to other games.

They can not stop creating new content and develop the game, because when they do this the game will start to die.
At the same time, more and more resources are needed to further develop such a giant project.

15 hours ago, Eimantas Gabrielius said:

But maybe is there any "key" which takes players to play this game for years

If I had to say the key would be making good content faster than 60% of your players can play it.

What made WoW good for me was that there was almost a endless amount of things to do, because the developers where adding in content faster than I could play it.

It also didn't feel like a missed something when I got to the new content late, because there where others playing at the same speed as me; made lots of online friends with that game.

WoW likely endures into the present day largely due to fans having invested so much time into the game that it's just become a part of their lives. The game's bled players recently, and the drastic transition from WotLK to Cata upset a lot of classic WoW players to the point that they stopped playing. It's definitely been "dethroned," and arguably has for a long time in countries like S. Korea with other, stronger MMO lineages (like... Lineage).

Something else to keep in mind is that MMO game devs primarily rely on casual returners, not just whales and hardcore fans. If you advertise content that draws players back for a month or few, then you've already gotten their money. Bonus points if you get them to splurge on microtransactions in that short period of time.