Something like 98-99% installed solar capacity in the American southwest is traditional PV. Mirrors are there, and they're awesome, but PV dominates.

PV are designed to account for heat and "less efficiency" means they risk performing at 17-18% instead of 20%. And it's actually generating more total energy at 18% because more total sunlight is hitting it, an advantage in desserts.

Yeah that's true.

I was thinking more long term though, deserts see much faster yearly degradation than places with more normal temps. (up to 2-3% compared to the standard 0.5-0.8%)

That's just an economic factor rather than a blocker. PVs are cheap as right now, and could be even cheaper if they weren't tariffed. I wouldn't be surprised if PVs in the desert is nonetheless the right approach right now, and not concentrators.