Um, that's like saying RNA polymerase has been around for a long time and hasn't changed...
I wouldn't say that Blizzard's expansions fail. They're the top MMO out there. Came out in '04 and still going strong - in fact, WoW has knocked down clone after clone and competitor after competitor. They just cater to a particular crowd. Which happens to be rather large. I'm not part of it, so I don't play the game.
About the whole content being released too soon - Riot does plenty of that. I remember off-hand several patches last year, one of which was the patch that had Pantheon's portrait revamp, that forced shutdown of ranked queue and unexpected problems. Victor had been disabled for quite some time, again due to issues. In short, Riot releases botched content all the time.
And what's especially troubling is that I do not hear/read any releases from Riot talking about the future of their game. All I see is an endless churn of tournies and new champion skins. And recycled, poorly-constructed and unimaginative champions. And more skins.
There is no talk of developing the game. There is no vision. I see a bunch of money-hungry shortsighted staff, just sitting on the current product, waiting until the game gets old and/or outdated.
That's what has me worried.