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 late 90's to the early aughts' race for highest-frequency, highest-performance CPUs exposed not a need for a CPU-only, highly specialised foundry, but a need for sustained access to the very front of process technology – continuous, multibillion-dollar investment and a steep learning curve. Pure-play foundries such as TSMC could justify that spend by aggregating huge, diverse demand across CPU's, GPU's and SoC's, whilst only a handful of integrated device manufacturers could fund it internally at scale.
The major RISC houses – DEC, MIPS, Sun, HP and IBM – had excellent designs, yet as they pushed performance they repeatedly ran into process-cadence and capital-intensity limits. Some owned fabs but struggled to keep them competitive; others outsourced and were constrained by partners’ roadmaps. One can trace the pattern in the moves of the era: DEC selling its fab, Sun relying on partners such as TI and later TSMC, HP shifting PA-RISC to external processes, and IBM standing out as an exception for a time before ultimately stepping away from leading-edge manufacturing as well.
A compounding factor was corporate portfolio focus. Conglomerates such as Motorola, TI and NEC ran diversified businesses and prioritised the segments where their fab economics worked best – often defence, embedded processors and DSP's – rather than pouring ever greater sums into low-volume, general-purpose RISC CPU's. IBM continued to innovate and POWER endured, but industry consolidation steadily reduced the number of independent RISC CPU houses.
In the end, x86 benefited from an integrated device manufacturer (i.e. Intel) with massive volume and a durable process lead, which set the cadence for the rest of the field. The outcome was less about the superiority of a CPU-only foundry and more about scale – continuous access to the leading node, paid for by either gigantic internal volume or a foundry model that spread the cost across many advanced products.
It's also interesting to note that back then the consensus was that you needed your own in-house fab with tight integration between the fab and CPU design teams to build the highest performance CPU's. Merchant fabs were seen as second-best options for those who didn't need the highest performance or couldn't afford their own in-house fab. Only later did the meteoric rise of TSMC to the top spot on the semiconductor food chain upend that notion.