Signs you might start worrying about Microsoft's future include

...when the army starts buying macs for security.

Actually, I think the combination of virtual machines (because they allow you to run one OS for backwards compatibility and another OS for newer stuff that actually, you know, works, thus providing a migration path out of Windows) and web applications (because they only need a web browser to work and are much easier to deal with maintenance-wise on the client's side) are the more likely downfall of Microsoft. But that won't happen for a while. 5 years at the very earliest, likely longer.