I think the most important thing Sun got wrong was having no credible onboarding path for new clients. Why would someone invest in SPARC and Solaris when a generic x86 with Linux would do 99% of what the SPARC box would do, for far less? Why would someone who is developing their software on an x86 box with Linux care about deploying it to SPARC+Solaris? Or SPARC+Linux? Or x86+Solaris? There is a huge advantage in deploying to the same platform you develop for, which means, not having a credible desktop will hurt you in the long run.

The same applies to IBM's POWER and Z - how many green field projects are being built for them? Apart from weirdos (such as myself) who is even interested in that? And even then, I will not suggest building anything new on AIX or z/OS, or Linux on either, unless there is a really compelling (such as "we got the hardware for free", or "we really need instruction-cycle latencies to run inference and we can't match that with any other NPU") reason.

Don't get me wrong - I love using uncommon hardware with uncommon OSs (which expose bugs that are affected by endianness, or Linuxisms, or blindly assuming a string is ASCII, or that a file stays put while you use it).