I don't think it is necessary to have Big Sur released for all older models first, just that they have a version that they are happy to load onto the machines and ship them with. I'm sure I remember a couple of machines coming out with otherwise unreleased software, or certainly devices needing updates right out of the box because they were manufactured with earlier, buggy or security compromised software (one of the iPads, I think?).People, people, people! I can’t believe nobody mentioned a very fundamental factor in the timing of the release of the first AS Mac. Hello...? They are NOT going to release any AS Macs until Big Sur is fully cooked and released to the general public. Folks are mentioning the first two weeks of October, but again, you need Big Sur to run AS Macs and fully take advantage of all the OS capabilities on the new chips, including power management.
So the very earliest release would be simultaneous with the release of Big Sur. And now you have your release date!
But with Big Sur rumoured to release alongside the October 13th iPhone event, which is the earliest I could see any AS Macs launching, it wouldn't be a problem either way.