The mmo genre has only itselfe to blame for its decay.
Instead of concentrating on the strengths of the genre, social and deep gameplay mechanics that get people invested, everyone was chasing the WoW dream after WoW managed to blow everything and everyone out of the water with its themepark style gameplay.
Funny thing is that WoW started off alot more "core" then the braindead themepark ride it is today. Only with each passing expansion became it more and more the mcdonalds of mmos.
So what happened was that all the other developers saw WoWs monumental success... and decided that wows theme park shallow experience was the way to go and the ONLY way to go.
Not to mention that the games that followed WoW where ALLWAYS complete technical trainwrecks that where pushed out the door when the games couldnt even stand on their own.
Age of conan, massive content cuts, broken PvP
Warhammer online, a clusterfuck of broken game mechanics and balance issues (my personal swansong.. the game could have been so awesome... how can you fuck up a warhammer mmo?)
Elder scrolls online: a buggy trainwreck that barely offered anything unique or substantial to differ it from other mmos.
And those are just three examples from the long long list of WoW alikes.
WoW was the one hit wonder, build on a strong IP that had an allready established loyal following that had for some time wished to explore the world of warcraft outside the strategy games. And most importantly: They delivered. It was such a vibrant and ginormous world to explore, chuck full of (allbeit shallow) content that was surprisingly easy to get into.
Sure they had their flawed mechanics but flawed doesnt mean broken. Sure the pvp honor system for example was a catastrophe that only 24/7 grinders could reach the highest ranks of... but it worked. All n all WoW worked on release.. that is more then you can say about any other mmo that followed in its wake.
No one tried to be deep anymore, no one tried to bring the social aspects of an mmo back into focus. It became: Go here, accept 10 quests to deliver different kind of bear asses, go kill mobs to collect said bear asses, finish quests, go to next quest hub, rince repeat.
What people that looked for new mmos experienced all the time following WoW was: been there, done that. Also the mmo system of leveling up your character from lvl 1 to max lvl becomes so agonizingly painfull after a while. You can only take killing the same low lvl critters (mostly wildlife and the like) for so many games till you cant stand it anymore. In a FPS theres no level up, youre a badass from the getgo. In RPGs theres a story reason why your character starts out weak. In most sports games you dont have to grind to have your players be at a level they can compete.
Another big problem with mmos nowadays is that everyone rushed to the endgame, cause the endgame is all that counts. All the hard work put into anything that comes before the endgame is pretty much wasted. Even WoW has that problem:
If you think about it WoW has the biggest mmo world of them all... and barely anything of it gets used nowadays. People spend most of their time in the handfull of areas currently important for the newest expansion and thats it. There is absolutely no reason anymore to even go to any of the areas that came before it even when leveling alts since you can simply stand in one spot and que up the dungeon finder, leveling solely through dungeons.
So when people rush through a new mmo to get to the endgame, you know.. the only stuff that counts in the big scheme of things, most mmos are simply not ready to present a meaningfull endgame. Most mmos after launch dont even have endgame content for crying out loud because so much time was invested into the theme park leveling up gameplay.
See in vanilla wow back in the day this whole "rush to the endgame as fast as possible" wasnt present yet, it took people months to get to max level, and the few that rushed through the game where simply ignored by the larger crowd that was still out exploring the world and learning things. Meaning blizz had more then enough time to work on their first raids and when the numbers of max lvl characters increased to a significant amount the raids where ready. And they took ages to complete back in the day... molten core was hard work. (especialy before its inevitable nerf)
MMOs mistake grind for content and ease of entry with braindead gameplay. Allways standing in the shadow of WoW instead of trying to step to the side to get some sun themselves.
In short: the magic is gone because everyone was chasing the white rabbit down the rabbithole and got stuck on the way.