In the case of Microsoft, and any closed source software development house, the bus is metaphorical, it could be bankruptcy, it could be policy, it could just be management whim. But you are vulnerable, and frankly more vulnerable than you would be if you used open source technology. At least then you have choice, and are not subject to market forces, whimsy or the finances of an external vendor. If necessary you can maintain the codebase yourself or hire someone to do so on your behalf.
Ill agree with you one level however, simply because something is closed source doesn't make it bad, and a lot of people in the linux/open source movement behave immaturely when assessing the quality of a technology based on its openness, (just as many in the closed source world do the other way around). However I mostly mean this on the level that they pretend they have no lessons to learn, which is why certain types of software in their sector is quite frankly pretty damn bad.