A lot, I think. PA-RISC had a lot going for it, high performance, solid ISA, even some low-end consumer grade parts (not to the same degree as PowerPC but certainly more so than, say, SPARC). It could have gone much farther than it did.
Not that HP was the only one to lose their minds over Itanic (SGI in particular), but I thought they were the ones who walked away from the most.
Acquiring P.A. Semi got them Dan Dobberpuhl and Jim Keller, which laid a good design foundation. However, IMO, I'd lean towards these as the decisive factors today:
1) Apple's financial firepower allowing them to book out SOTA process nodes
2) Apple being less cost-sensitive in their designs vs. Qualcomm or Intel. Since Apple sells devices, they can justify 'expensive' decisions like massive caches that require significantly more die area.
P.A. Semi contributed greatly to Apple silicon, but the company has nothing to do with PA-RISC. In fact, their most notable chip before Apple bought them was Power ISA.
Not that HP was the only one to lose their minds over Itanic (SGI in particular), but I thought they were the ones who walked away from the most.