Well I just subjectively disagree - as apparently do the legions of professional experts at multiple distinct companies who research user interaction and design these interfaces.
I'm convinced designers have since long overthrown human-machine interaction experts, and usability consistently went down in the search for what's visually appealing and cleaner. I particularly blame Apple as a trend-setter in this, along with everyone that dumbly followed the fashion. No appeal to authority will ever convince me that disappearing scroll indicators are good for me, as I've been burned by them and I see less able people around me having ever increasing difficulty navigating modern interfaces thanks to similar innovations.
The "professional experts" are not interested in usability. They care only about sales and/or engagement. You can see this in every consumer product, not just software.
> An always visible scroll bar or indicator is conceptually simpler than one that disappears randomly.
An always visible scroll bar takes up a good chunk of screen space when you have multiple of them, and it's easy to develop intuition for what's scrollable and what isn't, just like we know what's right-clickable and what isn't.
It should be an option and it is, at least on Macs.
Knowing that a view is scrollable and there's more content to see is absolutely not an obscure edge case but a basic accessibility feature.
Relevant past discussion: https://news.ycombinator.com/item?id=20951580
Also relevant, what happens when the user doesn't realize more content is available: https://news.ycombinator.com/item?id=21353920