Itanium sounded the deathknell for all of them.
The only Unix to survive with any market share is MacOS, (arguably because of its lateness to the party) and it has only relatively recently went back to a more bespoke architecture
The common attitude in the 80s and 90s was that legacy ISAs like 68k and x86 had no future. They had zero chance to keep up with the innovation of modern RISC designs. But not only did x86 keep up, it was actually outperforming many RISC ISAs.
The true factor is out-of-order execution. Some RISC contemporary designs were out-of-order too (Especially Alpha, and PowerPC to a lesser extent), but both AMD and Intel were forced to go all-in on the concept in a desperate attempt to keep the legacy x86 ISA going.
Turns out large out-of-order designs was the correct path (mostly OoO has side effect of being able to reorder memory accesses and execute them in parallel), and AMD/Intel had a bit of a head start, a pre-existing customer base and plenty of revenue for R&D.
IMO, Itanium failed not because it was a bad design, but because it was on the wrong path. Itanium was an attempt to achieve roughly the same end goal as OoO, but with a completely in-order design, relying on static scheduling. It had massive amounts of complexity that let it re-order memory reads. In an alternative universe where OoO (aka dynamic scheduling) failed, Itanium might actually be a good design.
Anyway, by the early 2000s, there just wasn't much advantage to a RISC workstation (or RISC servers). x86 could keep up, was continuing to get faster and often cheaper. And there were massive advantages to having the same ISA across your servers, workstations and desktops.
He was a key player in the Pentium Pro out of order implementation.
https://www.sigmicro.org/media/oralhistories/colwell.pdf
"We should also say that the 360/91 from IBM in the 1960s was also out of order, it was the first one and it was not academic, that was a real machine. Incidentally that is one of the reasons that we picked certain terms that we used for the insides of the P6, like the reservation station that came straight out of the 360/91."
Here is his Itanium commentary:
"Anyway this chip architect guy is standing up in front of this group promising the moon and stars. And I finally put my hand up and said I just could not see how you're proposing to get to those kind of performance levels. And he said well we've got a simulation, and I thought Ah, ok. That shut me up for a little bit, but then something occurred to me and I interrupted him again. I said, wait I am sorry to derail this meeting. But how would you use a simulator if you don't have a compiler? He said, well that's true we don't have a compiler yet, so I hand assembled my simulations. I asked "How did you do thousands of line of code that way?" He said “No, I did 30 lines of code”. Flabbergasted, I said, "You're predicting the entire future of this architecture on 30 lines of hand generated code?" [chuckle], I said it just like that, I did not mean to be insulting but I was just thunderstruck. Andy Grove piped up and said "we are not here right now to reconsider the future of this effort, so let’s move on"."
Actually no, it was Metaflow [0] who was doing out-of-order. To quote Colwell:
"I think he lacked faith that the three of us could pull this off. So he contacted a group called Metaflow. Not to be confused with Multiflow, no connection."
"Metaflow was a San Diego group startup. They were trying to design an out of order microarchitecture for chips. Fred thought what the heck, we can just license theirs and remove lot of risk from our project. But we looked at them, we talked to their guys, we used their simulator for a while, but eventually we became convinced that there were some fundamental design decisions that Metaflow had made that we thought would ultimately limit what we could do with Intel silicon."
Multiflow, [1] where Colwell worked, has nothing to do with OoO, its design is actually way closer to Itanium. So close, in-fact that the Itanium project is arguably a direct decedent of Multiflow (HP licensed the technology, and hired Multiflow's founder, Josh Fisher). Colwell claims that Itainum's compiler is nothing more than the Multiflow compiler with large chunks rewritten for better performance.