That is one way things could turn out. On the other hand, when Apple started to introduce the iOS-7-style frosted glass effects, we got backdrop-blur in CSS, which handles the hard parts of achieving a similar effect on the web.

If this whole liquid glass effect catches on big time (which isn't out of the question right now), we might see something in the web platform that gives developers access to the platform primitives required to render it.

Why shouldn't there be an effort to make more OS UI elements available via HTML? There's no technical reason against it. So I'm not saying that is what's going to happen, but I don't think this is a kind of moat they created with the explicit intention to make web apps worse.