I agree mostly; one of the tricky areas is pages with split layout containers that are individually scrollable, where it doesn't make sense to use the page/body

I agree mostly; one of the tricky areas is pages with split layout containers that are individually scrollable, where it doesn't make sense to use the page/body level scrollbar to control that content. Another common case are smaller widgets or modules that have their own scrolling context. The other half of the issue is that native scrollbars, especially on Windows, are a huge eyesore in those scenarios because they disrupt the layout or conflict heavily with the app or site's branding in some way. Take any eyesore in real life and it produces the same psychological effect — scrollbars too often feel like they don't belong. Of course, many here (and myself) would argue that usability is far more important than branding. I think a happy medium would be to enforce a certain size and stay with fundamental usability principles, while allowing customization of colors / arrow images at most, probably. A good, usability-focused designer alongside their engineers would keep these things in mind, ideally, but alas — a great many experiences are driven by amateur-level professionals who are still learning about these blind spots, and favor aesthetics over functionality.