I disagree. For people that are writing as a form of personal expression, posting publicly provides a source of accountability writing in a journal doesn’t. I think it’s a tad cynical to assume everyone who writes publicly at a high volume has an agenda or ego — they may need to write for their own benefit, skill-building, catharsis or mental health, and the challenge and vulnerability of being open to others might be the only thing that meets that need.
Looking at Medium from a reader’s perspective, though, I very much agree with your main points in the article; there’s so much you could possibly consume on here that it can get pretty exhausting.