The small width on user trying to use it is inexcusable, but hiding is a nice option as in too many cases the scroll indicator is not very important to be that visible (it's cleaner UI, though that shouldn't be mandatory either)
Windows 11 is about 1/3 the width of the above picture
They're mostly a relic from back when mice didn't have a scroll wheel (think Windows 3.1-era times). Nowadays, some people use them on trackpads because they either don't know how to do a two-finger scroll or a specific trackpad's just too shitty to handle it.