Yes - as someone who earns from ads on blogs - even from display ads (not ppc ads) - Medium could definitely earn significant revenue from placing ads on stories for non-members. That does 2 things - it gives some incentive to convert to a membership and it gives writers a chance to earn from external views. Some of my most viewed articles have large numbers of external views - which means no money for me, even though my content brought people here. I'd definitely appreciate being paid - and, of course, Medium would earn, too.