People who do some other stuff (heavy email users, some video/audio people) do sometimes use Windows (it's weird, but I think the Windows audio stuff is better for realtime now than Mac).
(the exceptions are people with FreeBSD, NetBSD, and Linux laptops)
I'd be pretty comfortable as a Silicon Valley employer only supporting Macs for office automation, and then either Mac or Linux for development workstations. If someone really wants Windows, s/he can support it independently.
The harder problem is phones -- there are people who are religiously attached to iOS and to Android, and you basically need to support both. There are pretty good MDM tools to cover both at the same time, but it does mean you can't push enterprise apps unless you do crossplatform development.