Of course, macOS and Windows 10 both still give you the option of creating user accounts independent of an Apple ID or MS account, which is what I do on my macO

Of course, macOS and Windows 10 both still give you the option of creating user accounts independent of an Apple ID or MS account, which is what I do on my macOS and Windows 10 systems. But there's good reason these OSes tie local logins to online accounts. The average user is more likely to get frustrated forgetting or not understanding why their email password is not their login password, than the (comparatively) rare scenario that someone will compromise the one-account-to-rule-them-all and wreck all their data. My grandmother confuses her Gmail login with every other online account because they all use the email address as a username. Also, I'm continually amazed how little normal people care about the data on their computers. I still have all my files from when I was 5 years old on my main machine, but most people only care about bringing over whatever they're currently working on when they get a new machine.