IMO, superhero comics have nothing new to say story or craft wise and haven't for a while. There is just nothing left after 80+ years. It's all run it's course and then some.
So I got to wondering when would I be comfortable with Marvel just cancelling their lines and calling it a day?
To be clear, I'm not looking for a date for Marvel to end where it would go out on top at it's zenith. That of course would be somewhere in 80s after all those legendary runs.
I'd look for a year by which Marvel has run it's course and ends up on pretty solid ground; a little exhausted, but still standing proud.
Specifically, the end would be Avengers 56 from 2002 which was Busiek's last Avengers issue. Or I could push it to FF 524 from 2005; Waid's last FF ish. At these points (and with Whedon's X-Men) I'd say there is nothing left, but everything is still pretty good.
On an off day, I might say you could wrap it up with Ultimates Vol.2 #13 from 2007. But at that point I think overall the milk has curdled.
That's just my personal feel. How bout you?
***Good argument could be made that 2003 JLA/Avengers serves as good close out point for both DC and Marvel!
I believe there are still lots of good story opportunities. Unfortunately they get squandered because the comic publishers want to make a statement or preach a message a lot of the time these days. And they're not really about the story anymore. There's a lot more money to be made in merchandise than in comics. As for the stories in the movies or cartoons, they seem like nothing but filler with bright lights and explosions.
These characters are for all intents and purposes immortal and their continuities are quite malleable. Lots can be be done with them.
EDIT: To answer your original question, I would say somewhere around the turn of the century for sure. It's been steadily downhill since with only a few bright spots.