This isn't really a constraint of MMORPGs. FFXIV brings in big shakeups all the time. This is more a problem with how Blizzard handles storytelling. It's taken almost a decade for any more meaningful storytelling to take place in Kalimdor and the Eastern Kingdoms after the Cataclysm revamp, and even then it's just another series of shock-and-awe set pieces that are set aside once it's done. Even the Burning of Teldrassil had so many plot holes it's amazing the tree didn't fall through the world before Sylvanas could torch it, plot holes which were acknowledged and blatantly brushed aside with, "That's not the story we want to tell."
WoW's problems with storytelling stem entirely from Blizzard's own willful incompetence, a level of blatant disregard for continuity that even Marvel and DC would shy back from.