You just scroll a tiny bit and you can see. Doesn't need to on the screen all the time, taking up space, distracting from the content which is what I care about, rather than whizz-bang user-interface elements.
Me asking myself "how long is this article" before I start reading it is a distraction? Too bad, I can't help but want to know. I could've learned that instantly by unconsciously glancing at the scroll indicator, but instead I have to move the whole content down for it to appear (possibly with a whizz-bang animation, no less), introducing friction and further distracting me from the content. Good job, I guess.
I don't know what you tell you - most people don't think about user interfaces like this. As we can clearly tell by people not designing them like this anymore.
As far as I'm concerned, the people designing modern user interface styles are not doing it for my benefit.
They're doing it to sell adverts (removing the boundaries between content, forms and advertising), track where I'm looking (he's opened the scroll bar! our content is engaging/boring!), adopt fashions to make their competitors appear dated and justify their own careers.
More than that, it's totally adversarial. I don't think the interface on a typical website is just sloppy or badly designed; it's designed to hamper access to information in a very sophisticated way, in order to prevent people from getting what they are looking for and leaving, but at the same time keeping them on the hook with the perception they are almost there.
If I were to find a causal relation, it would be the opposite: people not expecting affordances such as scroll indicators would be a result of designers hiding them in the first place.
Most user interfaces are not designed any more with any intent to provide what's best for the user, because they are part of a product being sold to corporations/advertisers.
