I agree, scrollbars are too thin (and there are many Blender users who do think so too):
- Change scrollbar width and visibility in Blender 2.8? link
- Please give users an option to switch to the classic non-hiding fixed-width (wide) scrolbars: link
- Change scrollbar width and visibility in Blender 2.8: link
- The super thin outliner scrollbar! link
… 
Scrollbars Design: https://developer.blender.org/T69391- UI Experiment: Scrollbars: https://developer.blender.org/D6505