Not to mention, if Bette ends up having an alternate, they aren't going to cast someone who is more of an event in the role than her. Bette is a bigger star than Carol, of course, but we're she to do it the story of the revival wouldn't be Bette Midler, it'd be Carol Channing is back at 95.
I am a firm believer in serendipity- all the random pieces coming together in one wonderful moment, when suddenly you see what their purpose was all along.