Earlier quoted context omitted.
I run a bootstrapped SAAS business and I make far more than $20K a month profit and have done so for years. The key is choose a niche that is large enough to make the level of profit you want, but too small to attract VC funded competitors. The sort of market size you want is in the range of $5 to $20 million per year. Big enough to be worth the hassle, but too small to attract the big guys.
What's your point? Twitter isn't in that niche, obviously.
Even that One Big New Thing they announced, namely Moments, still operated exclusively in that short messaging pubsub niche entirely (and turned out not to be something their users cared about that much so far).
Now their niche market seems kind of saturated, user numbers mostly peaked, and Facebook also started competing even more in that niche when they introduced searchable #hashtags and ways to follow accounts without becoming mutual friends, while still offering their other core features and "external" services like instagram (in direct competition to twitter) or whatsapp (in direct competition to twitter DMs). Tumblr is still there, which to me always felt mostly like a "twitter" for longer-form messages. I saw people switch from twitter to Slack (and recently Discord) and embrace their not-globally-visible pubsub models in some cases, so there is some competition there too.
So in conclusion: twitter seems to me like a pretty niche service (albeit large niche), and has competitors left and right now. It isn't in "that" niche, but in a niche.