Only when something better comes along, that is easily available to everyone who wants it. And right now there is nothing on the landscape that jointly meets those two requirements.
OS X is easily available through the Mac stores and, as for Linux, you can either grab an ISO or, for the less technically inclined, request a free CD to be shipped to you. The alternatives are available and they are building momentum. Even vanilla BSD (i.e. Free/Net/Open BSD as opposed to Apple's souped up version) is trying to get a piece of the action. Moreover, manufacturers are beginning to offer Linux preinstalled on systems other than servers.
Face it, Microsoft needs to innovate or they will be relegated to the dustbin. They won't die, they will just become a shadow of their former selves, much the same way that IBM and Novell did (and may remain, only time will tell). I am not saying either Mac or Linux is the next big thing, because that would require knowing the future, something I don't know, but the time has come for something new and people are beginning to look for it.
Go ahead, quote Microsoft stock prices until you are blue in the face, but the truth remains that things are changing. Even Microsoft has been forced to acknowledge this implicitly, to some degree. Once, the only way to get their development tools was the professional packages or the slightly cheaper, but still hefty, academic prices. Now, they are offering express versions of their products online. Why? Because the hobbyists that Gates has so despised in the past become professionals and they like to bring their tools with them. If those tools are PHP and vim rather than Visual Studio, Microsoft is toast. Or the fact that IE's standards support has gotten at least a little better. Even though Firefox hasn't taken over, losing a few percentage points in market share has been enough to shake them into the realization that they have to pay at least lip service to standards. Or, for example, the fact that they got ECMA standardization for both C# and the .NET CLI, or their failed bid to get an Office XML format standardized. Microsoft may still talk a bold game, but at least some of them know that something needs to change or they will start losing real market share.