> Or was it the gradual phasing out of mainframe-class hardware in favour of PC-compatible servers
Proprietary Unix is still around. Solaris, HP-UX and AIX still make money for their owners and there are lots of places running those on brand-new metal. You are right, however, that Linux displaced most of the proprietary Unixes, as well as Windows and whatever was left of the minicomputer business that wasn't first killed by the unixes. I'm not sure when exactly people started talking about "Enterprise Linux".
Back then I went with Debian, but I agree - the early scale-out crowd went mostly with Red Hat. Back then there was a lot of companies still doing scale-up with more exotic hardware with OSs like AIX and Solaris.
I first remember the term when Oracle ported themselves to Linux, began submitting patches, then began pushing Oracle on Linux to enterprises.
Oracle's big reason for doing so was because they could charge more for Oracle on Linux, and still get to a lower total cost of ownership than Oracle on Solaris.
Oracle began this in 1998. By 2006 they had their own Oracle Enterprise Linux distribution.
Proprietary Unix is still around. Solaris, HP-UX and AIX still make money for their owners and there are lots of places running those on brand-new metal. You are right, however, that Linux displaced most of the proprietary Unixes, as well as Windows and whatever was left of the minicomputer business that wasn't first killed by the unixes. I'm not sure when exactly people started talking about "Enterprise Linux".