I thought about this too, actually mentioned it in a somewhat related article I wrote on this subject to one of the Medium staff. I think this could work but they would have to be pretty clever with the user interface to make sure it doesn’t get too complicated.
I think part of the reason everyone defaults to binary “on/off” switches is that it avoids any ambiguity whatsoever. If however you opted for a sliding scale or even 4 star rating system, my guess is the participation rate for Medium as a site would go down. Folks gets “analysis paralysis” even with that simple range of options.
That said, this should definitely be a consideration! In fact, i may add a post script to the article to highlight suggestions that have come up, will reference you in it!