You already mentioned two examples. But open sourcing a database doesn't just prevent against the folding of the company behind it. It creates a community that

You already mentioned two examples. But open sourcing a database doesn't just prevent against the folding of the company behind it. It creates a community that drives improvements and helps prevent the project from folding. Clickhouse for example has a growing community that files bugs and authors improvements. CloudFlare for example contributed some very useful features. Open source also means one can examine the bug tracker (in most cases, some don't provide an open bug tracker) for known bugs and dive into the implementation in order to understand the inner workings in more detail if needed. I've made good use of this ability numerous times in the past. The gist is: if a database which is always a key part of a software architecture, is not open source, then it better provide extremely convincing arguments to choose it over other products. Being open source on the other hand doesn't mean choosing a partical software is a no-brainer. There are tons and tons of open source databases with questionable quality. Open Source as a criterion is just one amongst many. I would for example not hesitate to use some hosted proprietary DB on AWS if it fit the project because I know AWS is unlikely to go away. But some smaller/young companies? The risk that they'll disappear unfortunately is very real in this industry.