There are so many reasons it's not easy or straight forward to open source the software of a startup that failed. I don't know what to tell you, I guess since I've only worked for startups I kind of understand what risks are involved when working with them, but there are also benefits, it's a trade off that you should be making when you're informed.
If you want stability, buy from a profitable and stable company that can pen multi-year contracts that include liability clauses, and if your company has the leverage, get source code/on prem written into your MSA.
All that said: please, don't use new software or tools, avoid startups and steer clear of emerging technology, you'll feel less betrayed.
*I'm not a founder, never have been a founder, and never could be a founder.