It hasn't stopped WoW.
In fact, endgame really probably holds very little draw to a very, very large portion of the customers for both games. As long as there's other little stuff to keep people entertained, they'll be happily hooked. Endgame does not a game make, as the people it's important to are probably quite the minority. A lot of people never even make it through the journey and are quite content with it.
I am not sure what you mean, but WoW has more endgame then any other MMO before it..
and trust me, I am far from a WoW fanboi, but I can't argue the fact that WoW has endgame. It has a huge abundance of content to wade through.
It has an arena system that if you play enough could even make you some money if you really wanted it to (of course it doesn't really return enough to merit the time investment, but hey if thats your thing more power to you)
The thing is that with WoW when you get to the max lvl, there is still things to strive for.
With all the past failed mmo there really wasn't anything.
WoW has a huge culture behind it now, so many people decided they like it that it isn't just a game now, it is a lifestyle. It's pretty scary.
There is WSG and MLG that host WoW tournaments, as well as the official blizzard ones. That alone feeds the population of wow, and gives people something to push for; similar to how people grow up wanting to be a professional athlete, people are growing up wanting to be professional gamers; it is a form of entertainment for many to watch WoW arena matches, speculate on class balancing, strategy, etc.
The original guild wars was successful for a similar, but not as pronounced reason; first off it had no monthly subscription which is what brings people in to try it out; then they had a ranked arena system before WoW did, which allowed people to compete against each other in a more tangible way then just "My guild is better then your because I said so".
Now it is "My guild/team is better then yours because we have a ranking system that proves it is".
For the majority of gamers they are looking for some competition; humans naturally want to compete and try to be better then one another, that is how we have developed into our modern society.
And you may be thinking, well that only explains why people would play for PVP, but that is far from true.
For the same reason people compete for "world firsts" or "server firsts" with PVE content. There is no officially moderated blizzard rankings for PVE, but you can find a ton of sites monitoring all of the major guilds and their RAID progression. Of course it has stagnated now since there hasn't been any new RAID content for WoW other then Ruby sanctum in a while, which wasn't even a progression instance it was just extra filler.
Basically, if a MMO developer can successfully give people something to constantly push for, and compete against each other which, then the game has a good chance of being successful.
Of course that doesn't mean because you have done this it will be instantly successful. You need to cover the basics first, like.... is the combat system enjoyable? Does it work for PVP? Does it work for PVE? Is it balanced?
Age of Conan failed a several of those. In additional to having almost 0 content past the lvl 40 mark.
WAR was fundamentally flawed from the start because they tried to hard to make it like WoW, and failed miserably. The combat system was clunky and laggy, and just generally made you feel like "OK, so the combat is supposed to play like wow, but it feels like WoW combat programmed by a junior level programmer"
The whole "you can do scenarios from the start and pvp to level if you want" and "public quest system" was genius though, but it didn't save them from the fact that, if the combat system sucks, the game sucks.
Aion had a great combat system, but the combat system failed once you got into PVP because the use of flying.
Also the keep system for aion failed because the combat system did not really take into account zerg combat, which is what aion keep taking was.