In a hypothetical world where AMD wasn't allowed to do AMD64, and Intel stayed committed to Itanium: Itanium would still have sucked, and both PowerPC and SPARC would have out-sold Itanium by an even wider margin than they did in this reality. Itanium could only have succeeded if AMD64 wasn't possible and literally all of the competing 64-bit architectures were killed off by their owners so they could jump on the Itanium bandwagon. Itanium managed to kill off PA-RISC and Alpha and (high-end) MIPS roadmaps, but it still had competitors that were not just viable but actually more successful.
If you ignore VLIW, you just have a very unexciting RISC ISA, but because of the VLIW, you get extra scheduling info that most RISC designs don't provide which might be advantageous. The real question is actually about the code density of 41-bit instructions and if it can be offset by the 128-bit package (and perhaps something like allowing new 24-bit compressed instructions).
Poulson already somewhat proved part of this as it added back a traditional frontend and even added some OoO capabilities and 4-way SMT. It wasn't earth-shattering, but it wasn't absolute garbage either.
The contemporaneous IBM POWER ISA was implemented in superscalar CPU cores with out-of-order execution and with SMT and it would provide superior performance in any equivalent fabrication technology.
The Itanium ISA actually had a few nice features, but it also had other bad features that outweighed the good features. Besides the static instruction scheduling in bundles, there was also the handicap of using SPARC style register windows, which slowed-down context switches.
The second version of HP PA-RISC, which was too quickly replaced by Itanium, would have had good chances of providing superior performance in comparison with Itanium, had it not been abandoned without a fight.
Um...while not "OG Windows" I guess, PowerPC was an officially supported and shipping Windows NT 3.51 & 4 target[1]. It was released a couple of years before XP 64-bit for itanic. SPARC was a planned NT port, and while it never happened, it could have[2][3].
