At their scale, they will need to pay someone for support no matter what, whether that "someone" is Microsoft, Red Hat, Canonical, etc. So, you can't just look

At their scale, they will need to pay someone for support no matter what, whether that "someone" is Microsoft, Red Hat, Canonical, etc. So, you can't just look at the consumer price tag on a Linux distribution ($0) and compare that to what the IRS is paying Microsoft in support contracts - you have to compare it to the total costs of enterprise Linux support. And if the IRS is having a hard time migrating off of a version of an OS that was released 12+ years ago to its more recent version, I imagine they'd have a hard time migrating to a completely different operating system altogether. Even if the incremental costs of upgrading LTS versions of a Linux distro ended up being smaller, they simply don't have the up-front capital to cover the costs of migrating to Linux in the first place. (Oh, and don't forget that even if they did manage to procure that somehow, as soon as Microsoft gets wind of it, the price for Windows support contracts will magically fall).