Channing may have been too larger-than-life to play Dolly on the screen, but I think she would have generated the same amount of interest in the film as Babs did.
There is no denying that Babs was miscast, but it's also pointless to say that she was bad in it. She is fantastic in the film - and just because she is so different than Channing or Bailey or Merman doesn't make her bad. Obviously, she was too young. At 29 years old, what parade could possibly be passing you by? I also don't think that there is anything terribly wrong with making her Dolly Levi Gallagher rather than Dolly Gallagher Levi. It's simply a change in culture and origin, and it doesn't altar the message and charm of the story one bit.
The reason the film is bad is because of the overblown sets and the horrible pacing. It's incredibly long, where as the comedy and farce of the piece should move right along.
Other than Crawford, I find the supporting cast splendid. And yes, I even think that Barbra is wonderful in role, considering that she WAS painfully miscast.
"If you are going to do something, do it well. And leave something witchy."
-Charlie Manson