And even the comparison with RHEL is not really fair. New applications will continue to be compatible with Windows 7 for many years, while with RHEL you are mostly stuck with what you get with a particular version. EPEL et al. do provide some relief.

Suggesting that Xubuntu is a viable replacement is disingenuous - Xubuntu 12.04 LTS is supported until 2015. Let's take vanilla Ubuntu 12.04 for the sake of the argument. It's supported until 2017. A large organisation will not deploy a new system when it is released (many organisations are migrating to Windows 7 four or five years after the release). So, let's say that they deploy it now. That's only good for three years of support. So, in three years you have to redo the whole migration. Users have to be retrained now for GNOME 2 and in 2017 for Unity. You see where it is going, it is a mess.

Of course, then there is the issue that many existing Windows software doesn't work on Linux, compatibility of LibreOffice with Office documents, etc.

Linux on the enterprise desktop is usually not a viable option, except if you already have good infrastructure to support and maintain it (e.g. Google), or for a subset of users (e.g. developers).