Xeon Phi failed for a number of reasons, but one where it didn't need to fail was availability of software optimised for it. Now we have Xeons and EPYCs, and MI300C's with lots of efficient cores, but we could have been writing software tailored for those for 10 years now. Extracting performance from them would be a solved problem at this point. The same applies for Itanium - the very first thing Intel should have made sure it had was good Linux support. They could have it before the first silicon was released. Itaium was well supported for a while, but it's long dead by now.
Similarly, Sun has failed with SPARC, which also didn't have an easy onboarding path after they gave up on workstations. They did some things right: OpenSolaris ensured the OS remained relevant (still is, even if a bit niche), and looking the other way for x86 Solaris helps people to learn and train on it. Oracle cloud could, at least, offer it on cloud instances. Would be nice.
Now we see IBM doing the same - there is no reasonable entry level POWER machine that can compete in performance with a workstation-class x86. There is a small half-rack machine that can be mounted on a deskside case, and that's it. I don't know of any company that's planning to deploy new systems on AIX (much less IBMi, which is also POWER), or even for Linux on POWER, because it's just too easy to build it on other, competing platforms. You can get AIX, IBMi and even IBMz cloud instances from IBM cloud, but it's not easy (and I never found a "from-zero-to-ssh-or-5250-or-3270" tutorial for them). I wonder if it's even possible. You can get Linux on Z instances, but there doesn't seem to be a way to get Linux on POWER. At least not from them (several HPC research labs still offer those).