I'm not quite sure what you're referring to as "upgrades" for characters like Spider-Man. Sorry if I misunderstood your point.
But I agree with others that they need to evolve to keep characters fresh. Incredible Hulk has what, 700-800 issues total since the beginning? Would it still be good if it was all "Hulk smash" the entire way through? They went back to that, huh? It's still interesting with the Planet Hulk and World War Hulk stories though.
And sometimes the evolutions are just cool. Read Iron Man: Extremis. At the end,
Spoiler:
Stark finds a way to control the armor with his mind (the armor he keeps in his suitcase just flies to him), have the undersuit part of him to grow around him as he pleases.
It was just really cool.
Retirement?
I don't wanna lose the FF, Spidey, or any of the other greats because they refuse to evolve and are getting tired because of it. That's not fair to new readers either. They can pick up Painkiller Jane or Fall of Cthulhu (which I'm quite enjoying actually), but they would have missed the good times with Spawn, Iron Man, and Daredevil?
My nephew is 3 now and he loves Spider-Man. It'd be a damn shame when he's old enough to read comics, they decide to stop making Spider-Man.