If people liked it, that’s what we’d have. Surely this is a simple concept?

It's bullshit. Most people choose from among the handful of things the corporations offer them. You have to be exceptionally blockheaded to stay with an OS that no longer receives security patches, even if you prefer its interface paradigm, and if you're not the one controlling the machine you may not even have the option. The type of retrofitting I've done on my work machine is just that—work—and I understand why people may not want to do it, or may not be able to do it if they'd have to fight a draconian IT department for permission.

Furthermore, most people aren't designers or even terribly compute-literate. They don't necessarily understand which design elements are causing them to be so inefficient when they move to a different OS version, or how to revert them in cases where that's possible. They're stuck with Microsoft-Apple-Google's poor design decisions, until the same corp hands them another set of poor design decisions. The corporations don't want to decouple the UI from the OS the way Linux and other Unixoids do and let people choose, because the shiny new UIs are an advertising opportunity and impress certain types of reviewers.