> I'm curious did Itanium fail because the model of pushing the complexity onto the software and a human failed or did it fail because of lack backward compatibility for a world that was largely x86 at that point?

The answer is: yes

Itanium failed because getting instruction parallelism is actually incredibly hard to do and compilers didn't catch up in time to make it matter. But it also failed because of AMD64 which had backwards compat, was cheaper by a lot, and was a lot easier to get decent perf out of.

Opinion: I think Itanium was always destined to fail. The idea seems good on paper but fails in practicality because it limits what the hardware can do to achieve speed without breaking backwards compat with itself. You can't have a superscalar Itanium by definition because branch prediction etc. is by design delegated to the compiler. This means that the chips can literally only go up via clock speed, which as we now know doesn't scale forever.