Actors play lots of similar characters over the course of their careers. What makes costumed superheroes (and superheroines) different?
In a word, continuity. Harrison Ford may have been in
Blade Runner, a very different kind of sci-fi than
Star Wars, and Willis may have played McClane several times as well as lots of other cops, but those role's weren't ongoing franchises set in the same era. It's one thing for John Wayne to play lots of one-off cowboy roles several decades ago, but quite another thing for Reynolds to do two simultaneous costumed-hero franchises, especially when those roles might cameo in other costumed heroes' movies, as will have already happened to some degree if a Deadpool movie builds off
XMO:W in any significant way.
It'd be like Rob Lowe doing a tv show in which he plays a political consultant that isn't Sam Seaborn, or Shatner playing a retired space captain in some Wing Commander movie. The flip side to continuity along multiple films is a weightier actor/role association.
Besides, this is a different filmic era than Wayne's or even Willis' (at the height of his stardom, that is). I myself'd like to see it be a more democratic, inclusive one. And if that means diminishing old-school movie stardom in favor of more aggressively unique casting, so be it. Why not try more looks, more ethnicities, more attitudes, more
people? The talent is out there.
(Perfect example: one of the many reasons the Harrison Ford flick
Firewall utterly failed for me was that it took him nearly an hour to punch someone after his family is held hostage. It's Jones/Solo/Ryan, for Pete's sake; I just couldn't forget his usual persona enough to buy him as a milquetoast.)