Disaster. Medium should be a meritocracy and so stories must float or sink under their own steam. Generating an audience isn't just about getting followers or people to click on your stuff, it's about generating a relationship with people so they return time and time again. Sponsored pieces would throw a spanner into that.

If Medium wanted to do things a little differently, they could very much increase the requirements for curation and then offer writers a 'curation bonus'. That would drive quality up and that would help people get better as writers. More so than paying for a seat at the table. Mediocre writers shouldn't be able to game the system. The best writing should always have the biggest reach.

This was a fun thought experiment. Thanks :o)