Is it easier to switch from the LAMP stack or RoR to other platforms? Or Java to other languages? Switching costs are always high in technology, not sure what your point is. Not to mention there are no credible better alternatives to AD and Exchange except maybe hand over all and data to a third party with G apps.
If you want to switch from LAMP on Red Hat to LAMP on Ubuntu or even on Windows, it's easy because most, if not all, of the pieces are there. It's not so with Windows desktop or web apps - the pieces are proprietary and many of them don't run on anything other than Windows.
