Since we're comparing anecdotal evidence, for the past 10 years I've been contracting at all manner of large banks, finance companies, telcos, payment processor

Since we're comparing anecdotal evidence, for the past 10 years I've been contracting at all manner of large banks, finance companies, telcos, payment processors, and healthcare. They are overwhelmingly using Linux, and mostly Red Hat, in their data centers now. In the early 2000s it was mostly Solaris on Sparc on the back end, but in the last 10 years it has switched to mostly Linux on x86-64, with a smattering of Solaris 10 to run "legacy" applications. Of course they also run Windows, but it's only for Exchange, Windows file servers, and Active Directory for desktops. Your environment running only HPUX, Solaris, and Windows, is far from the norm. Linux is widely supported, and most importantly, cheap, compared to HPUX or Solaris (running on custom hardware like Itanium or Sparc).