Mr. Williams had pondered for a moment and replied, “I’ve been working on the same thing my whole life.”

Mr. Hong said: “It’s not a vanity project, it’s his calling. Other people calling it a vanity project actually tells me more about them than it does about Evan.”

That kind of determination may raise the odds but says little about long-term success. The darkest theory about the failure of Medium to catch on as an influential writing site comes from Cliff Watson, a 46-year-old advertising executive and onetime Medium contributor from Omaha. He thinks it is already a relic of an earlier era, kind of like communicating by carrier pigeon or telegraph.

“Medium feels like the perfect Obama-era platform: minimal, distinguished, self-important, elevated,” Mr. Watson says. “But as soon as the campaigns really ramped up their efforts in the primaries, we were living in a post-Obama world, metaphorically, and then literally. And that post-Obama world holds no room for minimalist, distinguished, self-important, elevated thoughtfulness.”

So what’s a mere writer to do? Mr. Watson says he is now working on a screenplay.

Even some Medium writers are critical of the new subscription effort, fearing they will lose whatever audience they may have had. “Why write for a pittance for your words,” wrote one of them, Keith Parkins, “only to then find no one is reading what you have taken the time and trouble to write, because it resides within a fenced-off ghetto?”

Medium maintains it is doing well by the metrics it cares about. The company says that the number of posts on the site quadrupled in 2016, and that it has 60 million unique monthly readers. After raising $130 million from investors, it had a valuation of $600 million last year.

Yet the path to significant revenue seems as elusive as ever.

Mr. Williams is dismissive of those who say making Medium viable is not merely difficult, but flat-out impossible. “That’s why I’m doing this, and they’re not,” he says.