Scroll bars have always been both control (press here to move around) and indicator (how much of this document is there, and how far through it am I?).
Always-on-screen scrollbars are only useless if you think that indication is useless. Many people find it quite valuable.
Correct, but you don't need the full thick scrollbar for the indicator function, some apps use a much smaller dot/guide
More to the point, the indicator doesn’t need to always be visible to be useful. Appearing only when the viewport is noving works fine on the iPhone, for instance, because slighly moving the viewpoint to reveal it is a convenient action, and less well on desktop PCs where your hands are less likely to remain near movement controls at all times.
It’s also applicaction specific: I care a lot more about relative viewport size and position when reading a long document than I do when I’m entering commands at the bottom of a terminal’s scrollback buffer.