I think being known for playing a superhero is no longer the problem for casting directors it was when Chris Reeve donned the cape. Look at how well the likes of Christian Bale (Oscar winner) and Hugh Jackman (Oscar nominee) or Toby Maguire have done in recent years. Cain and Welling, frankly, aren't particularly good actors and probably wouldn't have become any bigger stars than they now are even if they hadnt played Clark Kent or his alter-ego. Their careers aren't any poorer than many tv actors after they leave a well-known show. Routh probably suffered from only playing Superman once in a movie that wasn't particularly popular.