The evidence suggests this isn't AR prep at all. I watched Apple's 20-minute design presentation, and their design team makes the same point repeatedly: Liquid Glass has very narrow guidelines and specific constraints.
Here's the actual design problem Apple solved. In content apps, you have a fundamental trade-off: you have a few controls that need to be instantly accessible, but you don't want them visually distracting from the content. Users are there to consume videos, photos, articles - not to stare at your buttons. But the controls still have to be there when needed.
Before Liquid Glass, your least intrusive option was backdrop blur or translucent pastel dimming overlays. Apple asked: can we make controls even less distracting? Liquid Glass lets you thread this needle even better. It's a pretty neat trick for solving this specific constraint.
So you'll feel like you're seeing Liquid Glass "everywhere" not because Apple applied it broadly, but because of selection bias. The narrow use case Apple designed this for just happens to be where you spend 80% of your phone time: videos, photos, reading messages. You're information processing, not authoring.
Apple's actual guidelines are clear: only a few controls visible at once, infrequent access pattern, only on top of rich content. The criticism assumes they're redesigning everything when they explicitly documented the opposite. People are reacting to marketing tone instead of reading what Apple's design team actually built.
[1] https://peoplesgrocers.com/en/writing/liquid-glass-explained