Now the only remaining use case is when designers don't design components around default scrollbars. Sometimes the default one just looks too big compared to the size of the component. But this can be solved with design (don't cram stuff into small fixed boxes that then need to be scrollable).

What’s the difference between styling a scroll bar and, say, a button in an application like this? If you want everything to look how the user prefers and fit into the OS then you shouldn’t really style anything at all