The reason I'm not putting Apple in this race is that as a hardware vendor, it can't really scale to the extremes that are available to Google and Microsoft. Apple can (as it does with Macs) take the top 10 percent, but it lacks the resources to go much beyond that in a market operating in the hundreds of millions of units. Or, put another way, Apple struggles in the low-margin, mainstream sections of any broad market that doesn't have major subsidies (smartphones being the exception - tablets and iPods were narrow market products).
Microsoft's to Lose
Clearly the PC space is Microsoft's to lose and the key application defending it is Microsoft Office 2012 and, outside of Corel Perfect Office (which also runs on Windows), there really isn't a product that close to this well-entrenched productivity offering. Over time, however, Microsoft has under-marketed Office largely because it lacks true competition and the end result is that most users seem to take the product for granted. This means it is vulnerable because users don't really remember or value the capabilities of this product. Microsoft has moved Office 365 into a gap that is targeted at the midmarket to address the desire for a hosted Office offering, positioning it against Google's weak alternative, but it isn't yet in wide use, which suggests it may not be able to play the defensive role envisioned for it.
In short, Microsoft is exposed.
Google's Opportunity and Why It Will Fail
Google is currently cycling Android about 3 to 4 times faster than Microsoft is cycling Windows, which means, even though it started from behind, it should be able to pass Windows in a few short years in terms of capability. In fact, it does a number of things better today in terms of running on hardware that the next version of Windows will eventually run on and the migration process, from old to new hardware, is vastly better on Android than Windows today. In effect, all you do is log into a new Android device and your apps and data come down from the cloud and increasingly the apps are pre-configured. This last is the only shortcoming to migrations today as many non-Google apps still need to be configured after they have automatically downloaded on the new hardware.
However, Google has several things working against Android as a replacement for Windows - the biggest is it has the poorly received Chrome OS targeting PC-class devices and competing with Android for them. Unlike Windows, which will run on both, Google has fragmented its own efforts. In addition, even though it makes money off of advertising, it does it poorly and one of the reasons the Chrome OS was poorly received was that it was under-marketed. This is the problem with Windows Phone 7 as well, but in that case Microsoft is the underfunded challenger. You can't go after an entrenched competitor with a "build it and they will come" strategy and yet this appears to be Google's favored approach.
In short, while Google has a shot, it won't step up to take it and that is why it will likely fail.
The history of technology is full of missed opportunities. The Newton could have been the Palm Pilot; Origami could have been the iPad or Zune the iPhone; and Android could have been whatever takes the place of Windows. But if vendors won't resource efforts adequately then they shouldn't be surprised if they don't pan out. Currently, even though Google is out executing Microsoft in a number of ways with Android, it isn't resourcing the effort to a level that will win and therefore it won't. By the way, I could say the same thing about Microsoft and Bing.
Both Microsoft and Google have mammoth cash reserves, but both tend to underfund their efforts. I'm of the belief that you either do something well and completely or don't do it at all - evidently, in this group, I'm the minority.