HP, DEC/Compaq, and SGI all made the decision to drop their various bespoke architectures for Itanium years before prototypes were available based solely on what Intel claimed performance would be on paper. Even Sun and IBM made noise about doing the same thing. Honestly, I think it was inevitable that something like this would happen. By the late 90s, it was starting to get too expensive for each individual high-end server/workstation company to continue investing in high-performance chip design and leading edge fabs to make low-volume parts, so it made sense for all of them to standardize on a common architecture. The mistake everyone made was choosing Itanium to be the industry standard.

Yes, that's all true, but I blame HP more than the others because a large part of what went into Itanium came from HP. They thought that with their simulation results they would eclipse all other architectures with Itanic, and they were way off base, junking an architecture that had room to grow in the process. Even so, PA-RISC was still competitive at least into the days of Mako, though they kind of phoned it in with Shortfin (the last PA-8900).

IBM did ship a few generations of Itanium hardware, they just smartly never bet the farm on it.

MIPS and SPARC were always a little weak versus contemporaries, if SGI had forestalled a bit with the R18k that would have been enough time to read the tea leaves and jump to Opteron instead.

PA-RISC and Alpha had big enough captive markets and some legs left that got pulled out too soon. That paradoxically might had led to a healthier Sun that went all in on Opteron.