This is one of the more obvious theories, but my pal Dan Larkin said I should write it up, so here it is! In serialized comics, given enough time, comics will eventually regress to the mean. The "mean" in this instance is defined as what the average comic book writer has identified as the classic take on that particular character.
This doesn't mean that characters don't grow as characters, just that their circumstances/status quos tend to eventually regress to the mean.
You can have Julia Carpenter have an extended run as Spider Woman. You can even have a brand new character like Mattie Franklin get her own book as Spider Woman. Eventually, though, Spider Woman will be Jessica Drew once again, as Jessica Drew as Spider Woman is the "mean."
Luke Cage and Danny Rand are eventually going to become best buds again.
Hal Jordan is eventually going to become Green Lantern again.
Steve Rogers is eventually going to be Captain America again.
Superman is eventually going to cut his hair/lose the electric blue powers.
(Here's one that hasn't happened yet, but will, certainly - Bart Allen will eventually be Impulse/Kid Flash again).
Occasionally, there are changes to the status quo that are accepted as the "classic" take on a character, and won't be changed. Stuff like Swamp Thing being plants with a consciousness rather than a mutated human or Dick Grayson not being Robin anymore (heck, nor will the other original Titans go back to their old identities - no reversion to Aqualad). And while Grant Morrison might have attempted to revert to the original status quo for Magneto, the "mean" for Magneto has changed to the Claremont take, so you know that is the one that would ultimately show up again (maybe not as fast as it did, but still).
Not the most controversial of theories, of course, but hey, if Dan Larkin wants you to write something up, you write something up!!