Nothing will break. You just keep using the git backend if you want to keep being compatible with git.

> it’s tough to imagine why it’s worth pursuing a native and presumably incompatible backend.

Well, there's no active work on a "native" backend. There are basically three backends right now:

1. the git backend

2. A simple backend used for tests, you can think of it almost like a mock backend, you wouldn't use it for real work, but it's still useful as part of the test suite

3. the piper backend at google

There's not a lot of reason for anyone to produce another open source "native" backend, because 99% of open source projects use git.