The death of a behemoth can be quick or it can be slow. It seems we will see the slow version. That other OS for the moment is XP on about 60% of PCs. M$ will cut off support for XP SP2 in July. Some fraction of those PCs are still decent machines but for one reason or another will not move to SP3. Others will come to see XP as deprecated and examine choices. We know business is seriously looking at MacOS and GNU/Linux. Consumers and OEMs will be looking at choices too. The OEMs are in a pickle. If they switch suddenly to GNU/Linux they risk profitability in the tight hardware market. If they don’t OEMs that are efficient enough to ship GNU/Linux and profit will erode share/volume. M$ has OEMs over a barrel. OEMs depend on M$ for their margin.
The writing is on the wall. More OEMs are producing low-end stuff that is good enough for most of the world and consumers are having more choices. Those who shop on the web are free to buy inexpensive PCs running GNU/Linux. That market will grow rapidly. Businesses and governments will be able to buy naked PCs and supply their distro of choice. Since they have to re-image and all that anyway, they can save money by buying naked PCs. Better, they can buy thin clients.