I feel like this could be misleading, as the main branch has none of the guarantees that
stable has. It's released as nightly, and has to go through ~6 weeks of freezing & bugchecking as a beta release before it becomes stable.
For this reason, I would argue against the default branch being called
stable - it isn't stable.
Even naming it "nightly" would be relatively bad, though. I think there's an important differentiation between released versions and the in-development tree: released versions have gone through at least some vetting, and are frozen. Sure, the only vetting for nightly is "it's built", but that's still something slightly more frozen than the current development branch.
If it was the same concept as a release, I would agree with you that using the same wording would be good. But the development / main branch is a distinct concept with different connotations, and I'd thus argue that using the same wording is harmful rather than helpful.