Companies stop growing all the time: it happens all the time, and no company grows forever. A stock price is not just the value of a company's business today: It has future growth and risk priced in. What the stock market has been saying to Twitter is that the public believes that Twitter's ceiling is far lower than we thought in 2014.

In practice, this says that all the bets on hypergrowth were wrong, and the company must adapt: The stock grants that looked competitive before don't look competitive today, so top engineers leave. Staffing plans designed to keep a system operable at 10x of Twitter's current volume stop making sense, so projects change, and people are laid off.

So this has nothing to do with liability to promises (they've already IPO'd), but with the pain of becoming a less promising company. Twitter is too important to disappear in the near term; all you have to do is look at how big they are for journalists and presidential candidates to figure that out. There is, however, an opportunity for someone to make a competing product that fits the same needs and is easier to monetize.