If you think that's bad, try the NHS in the UK. Its the largest organisation in the world with 1.4M employees (!). They've been rolling windows 7 out for 4 years. It'll be obsolete when they've finished.
It's big, but it's not that big, fifth or sixth largest employer in the world according to Wikipedia:
http://en.wikipedia.org/wiki/List_of_largest_employers#Large...
Plus DoD is several branches so that's a bit of a push.
Maybe they should standardise on a particular UI for specific applications - maybe with desktop icons in specific locations for launching those apps. Then they remove the OS from user view entirely - any OS or version of an OS that will present the particular applications with the required views would then be appropriate.
That way when determining if they can upgrade to Windows 10 they just need to ask "will our current apps run, with largely coterminous views and function? Can the OS login direct to our launch icon layout?".
Most users use applications rather than OS, especially in a work setting I'd warrant.