Window's won't always have such a large market share. In fact, very few products have ever held such a large market share w/o government enforced monopolies. In a truely free market, such a monopoply is impossible.
Luckily for Microsoft such a "truly free market" doesn't obtain anywhere in the world and probably never will.
Microsoft's proprietary model doesn't have to be better than open source as a way of making software. All they have to do is make it too painful to create software around an open source model. You can bet that they're spending a lot of time and money figuring out how to do that. ESR is declaring victory way too early.