The vast majority of companies can barely build their own products, let alone study the source code of complex 3rd-party software. A distributed SQL database is on the extreme end of knowledge required to even understand it, so I'm not sure how open-source is going to help you.
As a counter-example, rethinkdb is open-source but the company failed and nobody cares about using it anymore. What would you do with that? Start building new database features yourself? Or just get your data out and move to a different system?
You will be able to at least maintain it, fix bugs, security issues. Maybe even start working on new features, promote your fork, revive some of the community, find people with relevant expertise, etc.
Databases are so lock-iny and critical that it's only natural for closed source database startups to be considered too risky to touch.
Possible doesn't mean realistic. As stated, 99.99% of companies are not going to come close to understanding, forking, and running their own build of a database.
It's better to practice proper vendor management and weigh all the risks and realities instead. If you're not more capitalized and viable then your vendor, then you have more important things to worry about then your vendor disappearing overnight.
/Widely used/ open source projects will find continued support; the user base of a project is a crucial part of the decision. Rethinkdb had only one serious site, who found it easier to port to a thin layer on top of Postgres when rethink collapsed.
As a counter-example, rethinkdb is open-source but the company failed and nobody cares about using it anymore. What would you do with that? Start building new database features yourself? Or just get your data out and move to a different system?