I'd say the problem is more that at some point, every site/service/platform becomes attractive for SEO/marketing purposes, bringing in tons of shysters looking for a quick buck. It's the same reason virtually every tech and webmaster forum on the internet is absolutely terrible; because all the spammers join and post crap en masse to try and build up a few backlinks/getting themselves seen as 'authorities'.

Just look at Sitepoint, Digitalpoint, virtually every web development and marketing subreddit, Warrior Forum, etc.

That's what killed Medium. Everyone started promoting it as a get rich quick scheme/easy way to market your work, so the spammers, marketers and morons came in.

The only way to avoid this issue is to heavily moderate the platform with a zero fluff/spam tolerance rule, and to boot out anyone trying to game the system.