In the rolling-release model that most browsers follow these days the alpha -> beta -> stable cycle is short so the typical "alpha slowly gets more stable & when it gets stable enough we mark it as beta" model doesn't and cannot apply here.see more0

Also, since MS uses Canary/Dev/Beta, Beta has a concrete meaning and using that name to describe Dev would be misleading at best.

No; beta is a separate channel that we're still waiting for. Dev is another alpha, just updated a bit slower than Canary.