My apologies, I did indeed mean Win32 rather than something as generic as x86, I'm getting long in the tooth and I sometimes forget that computer architecture is slowly standardizing.
While I don't disagree in principle with your assertion about existing multiple platform support in a business sense, I was more talking about Windows plus Mac plus Linux/Unix. I'm a big believer in the SOE/MOE model, primarily because of the cost savings on the support side of the house. You can often sign a single hardware lease contract with some hefty savings built in for owning and maintaining a large fleet of computers with consistent configuration. The savings stack up because you only require one image (or at most a small handful of them), which cuts back on your data storage requirements, image management tasks, software packaging and patch testing & deployment. You can also have a very specialized team of support agents who don't rely utterly on your knowledge base to provide support. You also save money at the point of service request, because your support desk agent knows that the hardware is pretty much standard. They don't need to spend time chasing details on device hardware, OS and software specifics, along with changing mental gears to think in terms of the specific hardware or OS.
I'm aware that you can use technology to bridge many of those gaps, but the time spent implementing those technologies can directly be converted to money, so it still counts. There's also the fact that if you've dropped half a million or more on developing specific software tools for your business, you may have a hard sell with the board to invest additional funds converting that system in to a web based product on top of requesting the funds to invest in additional technology. On that basis, don't also forget the cost of training, when you suddenly need to provide it for multiple platforms (and often for the same software across those platforms, due to minor but important configurations).
Lastly, keep in mind that for many large enterprises, especially in certain industries, cloud based computing is not a viable option. I highly doubt you will ever find a Google mail appliance supplying comms for Government departments, especially in areas like health, education, law enforcement and defence.
I should also point out that I speak from a position of experience. I've worked in helpdesk and desktop support for tertiary education facilities that supported Windows PC and Macintosh environments, and I can tell you that every one of us in the tier 1 and 2 teams detested calls from users on the Mac environment. Admittedly much of it was due to the self-important nature of the callers, but it was also due to the time taken to adjust our thought processes for what amounted to less than 5% of our monthly call intake.