None of us really know what an actor is capable of until he or she gives it a try. With MC's history of comedy, and Barnum as his best-known musical pre-Phantom, I'm sure his casting as the Phantom was incomprehensible to some. Yet he turned in a legendary stage performance.
(That said I don't approve of the casting of "stars" who can't sing, can't dance, etc. in roles that require same. We know to whom I am referring!! The important factor is to have a sane and objective person doing the casting..."can this person REALLY perform the role, or are we casting him/her for entirely other reasons?")
If you had ever seen Crawford in an interview, when he gets going in story-telling, you would know he can do over-the-top comedy beyond anyone's expectations. That said, he would be totally different from Nathan Lane, and some may not be able to accept a different characterization.