I dont understand your logic.. How could being prone to virus attacks be desirable for Microsoft? From a brand image perspective it is very bad. This is one of the major selling points Apple has over Microsoft, that OSX is less virus prone.
If you are a commercial software developer, you would like an operating system where security risks discourage users from pirating your software.
Yes, there's a limit to what users will tolerate. A system that is too insecure would not work. But the virus threat needs to be real to discourage piracy.
Ultimately, users do care about an OS with lots of software available. Many users will put up with some security threats to get that.