You don't understand what Microsoft provides if all you can think of is Windows.
Enterprise is where Microsoft's bread is buttered and until Linux or Apple can provide the wide variety of products that Microsoft does there is little chance of uprooting it even in the case of Microsoft bankruptcy.
Most of what Microsoft provides to the enterprise market is solutions to manage its Windows desktop solutions. AD, Exchange, etc... they're not unique in the market.
If Windows were gone, no need for GPOs, no need for MSIs, no need for Terminal Services, Exchange can be replaced by tons of other groupware solutions, etc...
Aside from that, Microsoft's enterprise offerings are pretty sub-par. Microsoft Cluster Service is crap compared to things like Symantec VCS, HP MC/Serviceguard, SteelEye's LifeKeeper. SQL Server can't really beat Oracle for bigger installations and for smaller ones, PostgreSQL and MySQL are definately the cheaper and easier to manage choices. Let's not even go near IIS, Apache simply kills it.
As for internal tools and software development, J2EE is still the biggest name out there, .NET doesn't even come close and its future is now even uncertain.
Anyway, Windows will be suffering a bit, even in enterprise, in the near future. Ever hear of BYOD ? That's the new trend. Enterprises don't want to bother users with standardized tools. Bring Your Own Device, whatever you're comfortable with. And if we look at the consumer market, Apple is pretty much the choice of what people are comfortable with at home. I bring my Mac in to work often as I much prefer working on that than my provided HP EliteBook.