We could potentially enable clustering for the Open Source version. One of the reasons it isn't already is that our clustering implementation is currently AWS-specific, since it relies on the AWS metadata endpoint to look up a host's internal IP, as well as networking rules that allow hosts to talk to each other.
This is why Vault requires another piece like Consul (plus a whole lot of tricky infra/networking work) to achieve HA.
That said, we could allow users of the Open Source version to specify a url via an env var to look up a host's internal IP so that clustering would work.
Auto-scaling is provider-specific though, so I don't see how that could be baked in. Same with secure networking.
I'll also just say that while we do want the open source version to be fully functional (if a bit more DIY), another motivation for us that I see as equally important for a security product is transparency.
While it's inarguably crucial for any clients implementing end-to-end encryption to be open source, I think there's a lot of value in open sourcing the server as well (regardless of how practical it is to actually run) so that users can know what's happening on the server-side, see that the code is high quality and tested, and so on.