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.

Paper-Itanium is unfairly blamed / credited with that, but it's not exactly true. The vertically integrated RISC/UNIX minicomputer companies were not on economically viable trajectories. x86 systems were undercutting their high margin workstation and server business, subsidized by enormous volumes in PC market needed to sustain increasing silicon manufacturing and design costs of new generations. The RISCs were forced to keep retreating to higher margin, lower volume, more specialized, higher performance markets which was really their only option by that point but it was also exactly the wrong direction to go in for long-term viability in the face of these prevailing trends.

Intel had their foundries and CPU cores paid for by video gamers and receptionists and students typing up their assignments and people looking at porn at 14.4kbps. Servers and workstations were pretty much gravy and every year there was less their systems couldn't do compared to the big iron.

Those computers pretty much sold themselves, contrast with every $20k workstation or $100k server that the unix guys had to explain why they were worth 5x the grey box on the desk.

SGI was struggling to make their MIPS chips and were pretty quickly finished off when nvidia started making GPUs. Itanium originated at HP as a PA-RISC replacement. They owned DEC Alpha when they killed it but arguably DEC didn't have a big base of legacy software at that point. IBM and Sun stood for a lot longer, mainly I would say largely due to locked in customers/data on Oracle and DB2 databases and proprietary software, rather than the merits of their hardware.

If Itanium never happened, 64-bit x86 server chips would have done the same thing to MIPS, Alpha, PA-RISC, SPARC, PowerPC. Might have happened even sooner from Intel since they allegedly sandbagged efforts in that direction to protect Itanium.