That's a relatively new feature (from November 2018... wow, has it been a year already?). My guess is that they implemented this stuff before that existed or maybe near to its release.
I have not used origin failover either, though I'm pretty sure you're right that this is its exact use-case.
You are correct, this has been implemented before Origin Groups have been released and they might be a viable alternative, but we haven’t tested them yet.
My concern is you (contentful) are leading folks down a bad path. Adding complexity, code, cost and a larger surface area of services that need to be up.
I have not used origin failover either, though I'm pretty sure you're right that this is its exact use-case.