I'd say that the cracks were showing in the late 80s. By that point you had to be a scholar to keep track of everything that had happened in the MU.
Many of the writers valiantly tried to keep some sort of continuity going up through the early 2000s, but by that point so many ret-cons had happened, stuff like Wolverine being involved in so many character's early histories that nobody could keep track of it. And the new generation of writers (maybe understandably) just stopped trying for the most part.
The days when you would get little editorial blurbs like "hey this story takes place between Spider-Man x and Marvel Team up y" were long gone, and having the same characters appearing in many titles a month become the norm. There was no way to keep track of it anymore.
Marvel's new stance seems to be that for the most part only the big past events in the characters lives will ever be referenced. Anything that contradicts non-important (in their eyes at least) stories is viewed as fine.
So the Marvel Universe still exists. It makes some attempt to reconcile big events of the past and modern continuity straight (say the last 10 years). Everything else may or may not have happened depending on whether a modern writer needs to contradict it to tell their story.
Which is a good or bad thing depending on how you look at it.