A Sequel?
This is the kind of movie that would be sequel-ed to death if it were done in modern day Hollywood.
I do think a sequel could have been done well for this.
There's so much appeal in the sad, ambiguous ending of Shane, wondering what he'll do with the rest of his life. Safe to say a sequel woulda somewhat spoiled that for lotsa folks (for first time viewers, knowing in advance that there's more to see would really dampen the climax's power).
I do agree that endless sequels would have had something of an audience though: there's always plenty of viewers who would gladly watch every single second of a character's life (or prefer a more definite end). Thank goodness Hollywood hadn't quite caught onto that yet.