The "developer" edition of Memsql does not permit you to put it into production and doesn't have any high availability features. So basically you can only run the enterprise version which starts at a minimum commitment of $25k annually and goes up according to RAM usage. Many small projects and startups will opt for a database that lets them get started cheaper/free and with less requirements. Also being not open source might make people worried as too many closed source databases folded in the past, leaving users stuck or forced to migrate to something else.
I evaluated Memsql for a project which would have made good use of both the row storage and columnar storage engines. But with Clickhouse, there's now a columnar database that performs extremely well and is completely free and open source. So half the usecase for Memsql went away. For the row based engine, the competition is a bit tougher. If one doesn't need extreme performance, CockroachDB provides a super easy to cluster consistent SQL db. And for people with more performance need, there's Mysql Cluster (NDB) for example or several NoSQL solutions.
Memsql is aiming for the enterprise market with well paying customers. They are not targeting the HN startup scene that much.
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.
I evaluated Memsql for a project which would have made good use of both the row storage and columnar storage engines. But with Clickhouse, there's now a columnar database that performs extremely well and is completely free and open source. So half the usecase for Memsql went away. For the row based engine, the competition is a bit tougher. If one doesn't need extreme performance, CockroachDB provides a super easy to cluster consistent SQL db. And for people with more performance need, there's Mysql Cluster (NDB) for example or several NoSQL solutions.
Memsql is aiming for the enterprise market with well paying customers. They are not targeting the HN startup scene that much.