I've wondered how Twitter would have been different if they'd taken a similar approach: $1 per account per year. They'd certainly be smaller than they are now, but I suspect they'd still be bringing in several hundred million a year before going the advertising route--the pursuit of which seems to be at the root of a lot of their questionable product decisions over the last few years.
None of that's directly relevant to the article, of course, which is implicitly about better filtering tools. I continue to be kind of bemused that this is so difficult for modern services to figure out, given that LiveJournal essentially figured it out fifteen years ago. Yes, the two services aren't directly comparable, but it wouldn't be wildly difficult to offer controls over, for example, who's allowed to @mention you.
IMO had Twitter tried to charge their demise would have been even faster than the current slow but steady decline we're all watching.
It's possible none of that would work, either, but I'm confident in saying there are at least tens of millions of people who do get value from Twitter. They've chosen a business model where having "merely" tens of millions of users may not be enough to support them, but I don't think that points to an intrinsic flaw in Twitter's concept--just an intrinsic flaw in their particular monetization strategy.
I don't think it's impossible to convert a large non-monetizing user-base into a profitable company (Google, Facebook, Snapchat, etc.) but I think that strategy is not a wise one for someone interested in building a sustainable company. There's a bigger reward to scaling before monetizing, but there's also a bigger gamble.
