No, it isn't. What you say is true in theory but it doesn't actually work in practice because major upgrades come with non-backwards-compatible firmware change

No, it isn't. What you say is true in theory but it doesn't actually work in practice because major upgrades come with non-backwards-compatible firmware changes. Try installing Mavericks on a machine that has been upgraded to Yosemite or later. (It also puts the burden on the user to actually make a backup before upgrading.)