In that case, another more proven RISC architecture like Alpha would have replaced x86. At that time, the only reason why x86 was still "competitive" was the enormous amount of x86 software (specifically Windows software). If Microsoft would have to switch to another ISA anyway, there wasn't really a reason to bet on something as risky as Itanium.
I mean, that happened and is still happening. ARM dominates markets that didn’t exist at the time, and is constantly chipping away and x86s strongholds.
For what exactly, it isn't as if Solaris was winning any desktop usage.
Microsoft and HP were already on Intel side, also Microsoft already had experience with JIT compiling x86 thanks to their collaboration for Windows NT on Alpha.
Yeah, but once Microsoft would have realized that Itanium was a performance dead end, and AMD wouldn't have jumped in as the saviour of x86, I bet Microsoft wouldn't give a shit about their good Intel relationship any longer and instead move to Alpha.
Had it not been the case and Intel alongside its OS partners would have managed to push it no matter what.