The basic problems:
1. barbra is too young
2. matthau looks like he swallowed vinegar
3. gene kelly was past his prime as an artist-director
4. the timing; this film was made in 1968 and 1969 at a time when the world was changing for the better and the worse
Can you imagine making this film in the midst of assassinations, riots in the streets, etc?
Even though too young, Barbra is the saving grace of the film. There is no way this film would have been made or been tolerable with Channing. Her voice, her look, her persona is better suited for the stage not for the camera. She is best in small doses; if she were a great screen actress she would have done more after the Millie film.
If BS did not star in the film, who would have been Dolly? Can you imagine a studio bankrolling a musical film in the late 1960s starring a woman over 50? [the same could be said for today.]
Do you know what happens when you let Veal Prince Orloff sit in an oven too long?