Without AMD rescuing x86, PowerPC wouldn't have died. MIPS wouldn't have died, and faces with the need to brake the 4Gn barrier on consumer equipment microsoft would have had windows running on three or four competing architectures until 2013 when everything would have switched to ARM.
Itanium would have already been a rotting corpse.
AMD rescued Intel from its own management.
WindowsNT is remarkably well suited to porting to other arch. This was done on purpose to hedge against x86. Dead one day, reactivated the other. MS needed to brake the 4Gb barrier and they would have done anything to get consumer priced 64bit chips.
Itanium was a dog. We had an Itanium SGI "supercomputer". Everything about it that SGI designed was amazing (hot swapping cpus). The cpu was a dog.
Without it, Intel would most likely kept iterating and improving Itanium's architecture.
The thing with AMD64 is that now there is this myth that there was no other alternative than keeping up with x86, and Itanium redesigns for some magical reason were impossible.
Other than those redesigns never happened, because AMD64 made economic invalid to pursue them.
I'll grant that it seemed like a decent idea at the time, but there was enough risk there that I can only describe Intel's all-in stance on Itanium to have been a blunder.
(There's a worthwhile distinction to be made between a blunder and a mistake. Mistakes are calculated risks that don't pan out, whereas a blunders is something that should have been obvious that it would go poorly when the decision was made. Losing your savings in the stock market from a surprise dip is a mistake. Losing your savings by betting it all on 14 in Vegas is a blunder. Similarly, the design of Itanium was a mistake. Betting the company on it was a blunder.)
One can imagine an evolution of IA64 that conceded microarchitectural complexity to allow adoption of a more compact instruction format with dependency tags rather than fixed bundle width, resulting in something more like an evolution of P6 with a different face, better x86 compatibility, and still ISA uniformity with the big iron. Or an evolution that involved buying Transmeta and adding IA64 support to that.
Obviously Intel didn't end up doing any of these things, probably because distribution strength and process superiority allowed them to do less and still post strong quarterly numbers. I'm not even sure if it was a blunder given the local incentives of the people who worked there at the time.