Many companies will agree to escrow their code (at the customer's expense) against the risk of them going out of business. I've never actually seen it happen, but it definitely has been offered when that concern was raised, and once the customer has to pay for it, they immediately say not to bother.

Imagine it’s a web or iOS or Android app. Imagine that you put 10,000 cards into supermemo.com and they took it down because of some legal dispute with another company. I have about 15k cards in anki for this reason: I’m not at a large risk of losing a significant investment if something randomly happens to the creator of the software

If it's not self hosted, it doesn't matter if you have the source or not

Good point. You could still import your backups into a replacement after somebody else resumes the project. Many apps don’t even offer backups or export. Supermemo.com incidentally did not at the time I moved away from it years ago