Keep in mind that we aren’t building standalone software, we are building an entire platform that isn’t targeted at self-hosting, open source won’t alleviate these problems. FaunaDB is solving operational complexity for you which is quite different from creating something for you to run yourself.
You could compare it to open-sourcing another SaaS platform, or open-sourcing DynamoDB or Spanner. There are similar questions for those to be found. And there are articles and opinions to be found on whether open-source in such a market is viable.
I can’t give you any information on what our opinion is though since that decision happened long before I arrived. But maybe, you are lucky and someone else jumps in. The question is though, why would you feel better about it? If it’s open-source it doesn’t give you any other guarantee than that you could look at the code.