In the unlikely event Charlie ever did get with TLRHG, he'd never internally see himself as measuring up to her. He'd be obsessed for the rest of his life trying to meet her expectations and would effectively treat her like a goddess. Charlie Brown wouldn't truly be happy unless he learned to be content with his circumstances. That applies to all aspects of his life, especially romantically.
Something to keep in mind as well is that we really can't know what the red-haired girl is truly like as a person. Charlie has effectively created a human idol in his head that even he isn't sure he'd still love if he ever got her (pic related). I truly believe that Peppermint Patty or Marcie is probably the best ending for Chuck regardless of if he can ever get TLRHG.