But this is partially missing the original point about proportional scrollbars, which is that they illustrate how large the scrollable content is at a glance.

But this is partially missing the original point about proportional scrollbars, which is that they illustrate how large the scrollable content is at a glance. With hidden scrollbars or, as I believe you're describing, simply not even looking at the scrollbar, there's no indication of where you are in the content or how large the content is. I would argue this is one of the chief grievances I have when consuming content on a mobile device (among an armada of other grievances with mobile consumption, at least on the web). Those of us who like knowing contextual information such as document size at a glance are sad to see (proportional) scrollbars go.