This is the reason for OS lock in the DOD. Billions upon billions of custom MS software exist in the DOD ecosystem. Cost of migration would be prohibitive. S

This is the reason for OS lock in the DOD. Billions upon billions of custom MS software exist in the DOD ecosystem. Cost of migration would be prohibitive. Second issue would be forest management. I am not sure which free or paid nonMS solution would replace the AD/exchange integration and the many other tools required to run a worldwide domain/forest with 2.5+ million machines.