But after that point, you see more and more (though not MANY) overdubbed singing for leads in movie musicals.
Well, something we should note - most of your examples are, well, kinda old. Guys and Dolls? West Side Story? We're talking about films made when 45 rpm records were still a going concern!
Now, musicals dropped off for a while, so there's going to be a gap, but I think if you look at the more modern incarnations - Mamma Mia, The Producers, for example, replacing the actor's voice is less common. I mean, Pierce Brosnan can't seem to carry a tune even when you electronically nail it to the bottom of the bucket for him, but they kept his voice in!
There is a significant value-add for having the actors sing themselves, in terms of audience reaction. When you're trying to justify greater and greater ticket prices, the authenticity starts to matter.