Again, this really isn’t a one-size-fits-all situation. With most ongoing comic book characters, they were created to be a perpetual thing (for as long as reader interest was there and the books continued to sell well enough.) That ongoing serial nature is a part of their appeal. At the same time, I can see the desire for an “ending”–it’s one of the elements that made Dark Knight Returns so successful when it first came out. But that said, I wouldn’t have liked to have made Dark Knight Returns the last Batman story and have missed out on all of the other Batman stories that we’ve seen since 1986 either.
Superman, Batman, Spider-Man and the like haven’t had any difficulty attracting the interest of new younger generations over the years–that’s a big part of why they’re still around. So you’re maybe looking to solve a problem that isn’t actually a problem.