MMORPGs are very expensive to make, and only very large studios can afford to have one fail. MMOs take around $60-100 Million to create.
MMOs have killed several moderately sized studios, and Blizzard would certainly have bankrupted without WoW's success. This means that people making MMOs must answer to a group of investors. This further means that most MMO creators cannot solely focus on the MMO. Investors don't invest into a game, they invest in a company. Games can be hit or miss, so they want long term stability. This results in several studios being forced to create several half-assed games while they focus on their MMO. This causes resources to be split, and ultimately cancellations/delays/failures to occur.
Alternatively MMO developers can partner with a publisher. However, publishers historically are very conservative and prefer to re-skin games that have proven to be successful. Which is why shortly after WoW, we saw a lot of WoW clones, like we see with any popular games. The caveat here, is this re-skinning doesn't work for MMOs. It works well for console games, because you beat the single-player, mess around with the multi-player, then move on. But MMOs are perpetual games.
MMOs are designed to consume all your free time. They are meant to be played to the exclusion of other games. Most everyone I know that really plays any MMO, exclusively plays that MMO and only dabbles in other games. People just can't sustain playing two MMOs at the same time. Furthermore, MMOs keep evolving, progressing, changing, and over the years you accrue a sense of being a veteran player along with a massive amount of rewards. Couple that with online bonds you form with friends, family, guild mates, etc, and it makes it very very hard to capture the attention of invested MMO'ers. A WoW clone is likely not going to make you give up 1, 5, 10+ years of online experience/rewards/relationships. It'd need to be something truly innovative.
To put it simply, WoW is massive, it owns everything. Unless people migrate to a new game - en masse - people won't really like the new game and simply just return to WoW. I've seen it with every WoW-killer. Rift, GW2, SWtOR, FFXIV, Aion, ESO, etc. People will leave for a while, then come crawling back to WoW. The market is probably saturated at around 15-20 million players, and WoW probably has about 50% of that. Not at the same time of course, but in the cyclical nature of MMOs.
WoW makes billions a year for Blizzard, and costs relatively nothing to maintain in comparison. It single-handedly funded ventures like Overwatch, HotS, Hearthstone, D3, Starcraft2, etc. So everyone is trying to copy it - which doesn't work for the reasons I mentioned earlier. It's too expensive to make for anyone to want to take any major risks, which results in WoW clones, which fails to pull a large number of players away from WoW because if you're choosing between WoW vs. WoW with Rifts or Lightsabers, you're going to pick WoW where all your friends/experiences/gear is. Slight mechanic/graphical changes won't make a WoW killer, because a WoW clone will never be better than the original. Everyone plays WoW, it has 10+ years of content, and you know it'll always be there. You never have to worry about all your effort going to waste.
The MMORPG will continue to live on in WoW, and whatever Blizzard makes to ultimately replace WoW. I doubt another company has the resources to compete with WoW in the way that it needs competition. All we'll see for a while is WoW clones, until Blizzard sweeps in with WoW 2 and dominates the market again.
Here's a
good video where most of my argument comes from
The future of MMOs will be in MMOs that aren't MMORPGs. MMOFPSs, MMORTSs, MMO-Racing, MMO-Brawlers, MMO-Puzzle games, etc.