There aren't many people who have bought MS software for years and are now excited to use a fork or their own build. Even assuming that some other mega corp comes along and offers world class support/service and their own fork, it still probably wouldn't hurt MS much.
Most small companies would have released their code as GPL (versus MIT as the case here) to limit the risk of a fork fragmenting everything (not being able to merge forks back in or make everything compatible). But in Microsoft's case, because of their size, few companies could hope to make a better version than MS. Further, MS makes money in lots of ways and this isn't a zero-sum game for them. They'll profit immensely from a more open ecosystem. Companies such as IBM, Apple, Google, and Oracle already have proven it's possible to be profitable with open source.
