In the first beta, macOS Big Sur declared itself to be macOS 10.16. But iOS' major release number is the first number in its name. iPadOS is the same, tvOS, watchOS; Just like when OS X became macOS - The Mac was an outlier. This is just bringing it into the same naming methodology as the rest. And macOS Big Sur is ushering in a new era. While it's not the OS that's responsible for it, we're changing architecture, which last time also warranted a name change, from PowerMac to Mac Pro, PowerBook to MacBook Pro, iBook to MacBook. Big Sur is the first Mac OS that runs on Apple Silicon, it's a new design language, though it does still heavily respect the history of macOS design. Big Sur is just evolving on things; It's not replacing the kernel or anything like when we went from OS 9 to NeXTSTEP; AKA OS X. But it also shouldn't. The Unix foundations of macOS show no sign of ageing, and on the contrary is a solid, composable architecture that can keep up with changing computer landscapes. While it fundamentally hasn't been replaced outright like when we went from 9 to X, the macOS today is not the Mac OS of early OS X. We have a new file system, a vastly different threading model, a completely new model for kernel extension-like behaviour that can run in user land through flexible kernel hooks, sandboxing and process isolation - And going up from the super low level, we have a new default shell, a new graphics API with Metal, a new language directly supported by the SDK in the form of Swift, UIKit on macOS, nearing a first-class framework, along with the declarative SwiftUI.
It's not an overnight overhaul, and it's a change here and there for a long time. macOS did not suddenly go from 10 to 11 like it did from 9 to 10. It slowly, birthday after birthday, became a version number older. I gave my Mac a little hat and a cake to celebrate. She didn't eat the cake so I ate it for her. But I think she's had a happy version number birthday anyway