Continuing stories of one character is challenging enough...but to do that for 40-60 years, all at the same time doing so in multiple books, it gets really dilluted.
I think it's come to the point where even the creative team realize that there's just not enough stories to tell with certain characters.
I think it'd be a good move for publishers like Marvel and DC to find a creative team to do one run and after they finish, they stop the book until a new creative team can be found to continue the book. Instead of just having a fill-in creative team to bridge the acclaimed creators, just put the title on pause.
Books like Ultimates and Astonishing X-Men will no doubt employ this method of continuing stories, so why not extend this kind of storytelling to your premiere titles?
Even TV shows take time off to create more ongoing stories.