>Interestingly, one of the reasons why Windows has done so well is because of the ecosystem it spawned. So whilst it might seem like a good idea for MS to do more, in fact it will only serve to destroy another part of the Windows ecosystem...
Yes, Windows has done well because it has spawned an ecosystem of productivity applications.
However, the idea that people choose Windows because it has a wider selection of anti-virus software than other OSs seems a bit of a stretch.