Legacy heroes can be fine, but the problem is that Marvel drastically changed too much in a short time. When a company drastically changes characters all at the same time that's usually when the problems start. I'll give an example: 1995. In that year you had:
-Peter Parker being revealed to be the Clone while Ben was the real Peter all along
-Reed and Doom were "killed" in 1993 and remained dead until late in 1995. IIRC Ben still had his scar from Wolverine until 1996.
-Captain America died but came back with the next issue.
-Tony Stark was revealed to be under Kang's control and killed some Avengers. Then he died and was replaced by a teenage Tony Stark.
-I think late 1995 was when Thor was banished from Asgard and depowered in Warren Ellis' short run.
-Punisher killed Nick Fury (yeah he'd eventually come back, but still)
-They were starting to get Daredevil out of the post-Fall From Grace period
-X-Men came off Age of Apocalypse but I can't remember what else happened during this time. Wolverine still had bone claws instead of adamantium, right?
-Hulk seemed generally stable unless you count the change PAD made the year prior where Hulk turned into a Savage Banner.
I can't remember what else happened during this time, though.