I pondered that question a couple of years back (see my post on the ENT Online site), and came to this conclusion: IBM would be hearing from the U.S. Justice Department, and the European Union, and every governmental entity in between. Monopoly on the desktop? Small stuff!
"Imagine if OS/2did trump Windows back in the 1990s. Imagine the monopoly we would becontending with -- far more serious than slipping in a free browser.With OS/2 Warp as the standard desktop operating and commodity serversystem, not only would one company would be controlling the OS layer,but also the processor, the box that houses the processor, the harddisk, the middleware, the applications, and even the integrationservices. IBM would have even controlled competing OSs, such as AIX andOS/400. Now thats a monopoly. The U.S. government, stategovernments and overseas governments would have been licking theirchops over this kind of antitrust action -- one that would havelaunched a thousand legal careers."
"...many are beginning to ask what was unthinkable a few years back: 'Where did the monopoly go?' While Microsoft has pulled strong-arm tactics to keep its VARs andend-user companies in line and forced into upgrades, there are choicesin the market. And IT managers are exercising their choices in droves... moving at least some applications from Windows to Linux environments."
Dana speculates that Linux may have either never come on the scene, or perhaps would have been actively promoted as an alternative by Microsoft. Let me add this thought that the US DOJ would have eventually splintered IBM into several smaller companies, which would have finally created an opportunistic opening for Microsoft Linux.
Of course, during this time, we don't know what direction President Gore would have taken his creation, the Internet...