Putting the feature into the Customizer makes sense, but I'm fairly sure that most of those who would use that feature were not creating child themes. People who create child themes don't use the Customizer by choice, but I'd never bother with a child theme for a few minor CSS changes. That's what Simple Custom CSS or the Custom CSS module in Jetpack are for.

I don't have user data (and would like to see some, because I always want to see real user data), but I'd bet that the people most likely to take advantage of CSS in the Customizer were far more likely to be using Jetpack's Custom CSS module, or some kind of plugin to let them adjust CSS without code, than to create a child theme, even a very simple one.

All of which is to say, the feature idea is interesting, but I don't think the headline reflects reality.