“Looks like only one of those is being maintained by a big corporation.”

I suppose the functionality would be pretty much the same by depending on MariaDB stuff. One could still connect to MySQL. For the Homebrew formula, though, I don't see a client-only Homebrew formula for MariaDB, alas. I see only the full package and a c connector, but maybe that includes the client library and CLI?

“Or maybe the other way around...make MariaDB the new core option and force people to use a special image for MySql?”

I'd be wary of breaking things unexpectedly for users.