That isn't really true. If Microsoft somehow ceased to exist, business would switch away from Windows and products written exclusively for that platform would be toast. The difference is really that for internet/cloud platforms, temporary outages can lead to lost revenue--which everyone developing apps on those platforms hopefully recognizes, especially given Twitter's former reputation for instability.
But the switch would not be immediate, a least giving providers some time to move to other frameworks. As you say, from a slightly different angle, it is the immediate effect that is the problem with relying on facebook or a similar network for your business: if they fall over tomorrow or simply change their rules so your product does not comply, your revenue stream stops that instant and until you can design and implement changes in the product or business model.
I doubt many businesses that have built and scaled their infrastructure on Microsoft systems would overhaul their entire operations just because Microsoft itself disappeared. Consider the amount of legacy systems still in use in large enterprises today; many of these were originally designed by companies that are long gone.
If anything, the collapse of Microsoft would produce many smaller companies competing to satisfy the ubiquitous demand for Windows support.
