> 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.
well, IBM very publicly invested 1Bn USD into supporting Linux on all their hardware with all their software. so db/2 on s390 on Linux, likewise websphere, etc. it gave the customers the promise of one run time environment on anything. and SUSE and shortly later Red Hat provided truly source compatible environments for software vendors. "code once run anywhere", for real. and then IBM and Oracle and Co forced suse and red hat to become binary compatible at the kernel/libc and basic system libs level, so Oracle and all could provide one binary under /opt on any Linux...
and that pulled all other vendors along, HP, Dell, Fujitsu, likewise for software...
and it all started with IBM officially supporting and pushing the hobbyist student project Linux on the holy Grail of enterprise compute, (of 1999/2000): s390
4.3BSD was basically designed without any bespoke hardware platform of its own. They commandeered DEC’s big iron, for the most part, to “dual boot” before dual-booting was cool. 386BSD began to enable PC-compatible hardware and really cost-effective “server farms” even before Linus was a twinkle in Finland’s eye.
Moreover, various BSD flavors were empowering admins to breathe new life into legacy hardware, sidestepping and bypassing the proprietary software channels. Linux, on the gripping hand, remained x86-unportable for awhile after BSD (and Xfree86) was running everywhere.
Personally I ran Minix-286 at home; at university we enjoyed a “recreational” VAX11 running not VMS but 4.3BSD.
Flash forward to 1998: from the arid but air-conditioned Sonoran Desert I received gently-loved twinned Apollo 425t systems with memory upgrades; I installed OpenBSD on both, as well as my 486DX100! It was a homogeneous OS environment with heterogenous hardware... and the 486, with Adaptec SCSI & a VLB #9GXE64 Pro, could boot into Windows 98 and run the Cygwin X server, or DOOM or Quake.
There was a golden age when a dude could walk into any surplus yard, grab Big Iron Unix Boxes, take them home and bootstrap NetBSD. On anything. Bonus: BSD originated in USA/Canada, for a trustworthy chain of trust. (Oh Lord, the encryption export technicalities...)
Did it though? Or was it the gradual phasing out of mainframe-class hardware in favour of PC-compatible servers and the death of commercial Unices?