A scrollbar is (also) an indication, glanceable, not requiring any action.
If someone among your friends and family has an iOS device, remember how many times you had to show them that certain screens can be scrolled down? Oh, not just down, but also to the right? There used to be a few perfectly aligned views that had no indication whatsoever that there's more to them is you scroll.
Narrow but visible scrollbars would solve this easily.