Right, but Apple didn't have to worry about the thousands of corporate clients running Enterprise software that relied upon some specific API calls to handle billions of dollars in business. When Apple says, "too bad, upgrade", their customers mostly just do so. If Microsoft were to take that approach, they'd lose billions in revenue.
My point, however, wasn't really that OSX isn't backward-compatible, but rather that I'm sure there aren't many engineers at Microsoft who wouldn't love to throw out the whole Windows paradigm and build something new and better from scratch. Hell, I bet BillG himself would have loved to Embrace, Extend, and Extinguish *nix. I think people have a tendency to view Windows' rather bloated codebase and adherence to outdated architecture as a failure of imagination on the part of its programmers, whereas it's really just a practicality necessitated by their user base.
I'm sure there aren't many engineers at Microsoft who wouldn't love to throw out the whole Windows paradigm and build something new and better from scratch.
Windows 95, 98 and Me are evolutions of the Windows 3.1 codebase, and then mercifully died. NT begat Windows 2000, XP, Vista, 7
You can see that they changed a lot of OS internals for the better (writing C++ code on win 3.1 it's trivial to lock up the whole OS, NT didn't have that issue), and then worked hard for compatibility.
While that's certainly true, the differences between NT 4.0 and 98 weren't particularly huge. NT was certainly the better OS, and it's good that they decided to adopt it, but it wasn't really a paradigm shift, and Windows applications were still cross-compatible.
My intention isn't to say that Microsoft hasn't made significant positive changes over the years, but rather that Windows would be a vastly superior operating system if the same people responsible for it currently were able to start from scratch. Say what you will about Microsoft, they still have some incredible engineering talent.
My point, however, wasn't really that OSX isn't backward-compatible, but rather that I'm sure there aren't many engineers at Microsoft who wouldn't love to throw out the whole Windows paradigm and build something new and better from scratch. Hell, I bet BillG himself would have loved to Embrace, Extend, and Extinguish *nix. I think people have a tendency to view Windows' rather bloated codebase and adherence to outdated architecture as a failure of imagination on the part of its programmers, whereas it's really just a practicality necessitated by their user base.