Microsoft has always prioritized backwards compatibility, knowing that it would get more and more difficult to get users to pay for an OS update if said update was going to break existing software, hardware, and drivers.

That, in turn, has largely prevented any meaningful updates in security.

When users worry more making sure that a five-year-old copy of ObscureWord or Doom runs without breaking, we end up with global BSODs.

BTW, Unix largely began as singletasking OS on non-networked hardware as well (PDP-7's.,11's).