Sure, let's do the math :
I only maintain a handful of desktops PCs, and 4 online servers facing the internet.
On those, replacing my Debian setup with a windows equivalent would cost me between 20 and 25 000 dollars _per machine_ in licensing fees, renewable every three years.
Also, Windows can't run fast enough on these machines (they're Atom based computers), so I would also have to pay for more expensive hardware.
I enjoy the full disclosure that open source provides, so I know exactly what's going on. Zero downtime in three years. _Very_ pleasant and efficient development environment (I started on Windows, so I can tell the difference).