I think that in an earlier age, Hanks would only have been a character/supporting actor.
Now, he passes for a Henry Fonda/Jimmy Stewart 'everyman' type, but back in the 40s and 50s, competing with them, a weaker-persona guy like Hanks would have been limited to supporting roles.
The 60s and 70s were all about hardened 'tough guy' actors like McQueen, Eastwood, Bronson, Coburn and Marvin or handsome ladies' men like Newman, Beatty and Redford, so there would be no place for Hanks among the stars then, either.
Only in the 80s and onward would Hanks have a chance. He came along at the right time for him, when overt masculinity was beginning to be expunged from Hollywood.