There are a lot of misconceptions about extreme heat like this in Phoenix. The majority of aircraft are not affected even by 120°F temps, particularly mainline fleets. Their performance charts go high enough, and the engines are more than capable of getting the aircraft off the ground. The issue is with CRJ200s in particular, but previously with MD80s and DC9s as well. The CRJ2s are underpowered enough as it is, and begin to struggle even on "average" hot days in Phoenix (110°F+). They're usually the first and often only fleet type to be grounded, but I think some of the other regional jets can face some weight restrictions.
Another misconception is that at those temperatures that runways and asphalt start to get "soft". Asphalt used in Phoenix is rated to temperatures in excess of 200°F, so an ambient temperature of 122°F simply wouldn't be hot enough to begin to soften the asphalt. Newly laid asphalt can, however, leak oil. That can give it the appearance of being soft, but the asphalt itself is not structurally affected. Aircraft were grounded on the 122° day in the early 90s because many aircraft performance charts didn't meet those temperatures, not because the runways or taxiways were at risk of being damaged.