I see this argument often, but I'm genuinely curious if it's actually the case - at least in my experience, a huge amount of apps nowadays have custom-designed UIs and very little conformance to "platform UI conventions". And that's even an expectation - if you see an app that uses standard UI controls, navigation etc - it comes off as basic and probably not really polished. I might be completely wrong.

On the other side, if users want standard UI and that's a factor in adoption, wouldn't people making PWAs then just make the apps in such a way - there's nothing stopping them, and there's no lack of UI libraries enabling that.