tl;dr: Microsoft has historically under-invested in security engineering, especially with regards to software architecture. I sort of agree. Microsoft's direct

tl;dr: Microsoft has historically under-invested in security engineering, especially with regards to software architecture. I sort of agree. Microsoft's direct competitors in this area (Google with Android, Apple with OS X) seem to be doing better than they are. But software security is hard, and it can't be solved just by throwing money at the problem; it requires a good development culture and practices, and in a company as big as Microsoft, building that is also hard. Microsoft is also saddled with considerably more legacy code and backwards-compatibility requirements than Google and Apple are; and SMB, which contains the relevant exploit, is absolutely legacy code.