How could this happen? I can think of three reasons.
mastodon.social is run by a nonprofit with a million dollar budget. It costs about $100K/year for hosting alone. They might just run out of money.
There could be a legal threat, and the service might not survive it.
Or the team might just go in a different direction -- building a PDS or a collaborative notepad or something.
I don't think any of these is likely, but none of them are impossible.