It wasn't really Itanium that killed RISC-based unix-like competitors, but just generally price. People realized you could do a shit-ton of clever things on a x86 platform, Pentium 3 class server on a $2500 1U or 2U rackmount machine. This is in the single core cpu era.

Running FreeBSD or OpenBSD or Linux and you didn't need to spend $14,000 on a SGI or Sun or HP or IBM AIX machine or similar. People didn't buy Itanium for the same reasons they didn't buy $14,000 Suns, except weird low volume enterprise customers. The death of the old-unix-like RISC machines in the market was due to broadly everything in the early i686-class Xeons, Pentium 2/3, early Opterons, etc.

Edit: this is my direct personal experience, but FreeBSD or Linux in 2002 was already far ahead of a legacy unix in terms of usability and ability to customize things. The legacy vendors were stuck in a model of software that literally came in a physical box on a CD-ROM and was rarely updated. And you had to pay out the ass for it. I remember having a FreeBSD system with KDE 2.x desktop in 2002 and it was far more usable than anything Sun or SGI.