Let's break it down.
Airlines are required to do performance calculations before each takeoff. One of the items used in the calculations is air temperature. If the current temperature is beyond the range of temperatures in the performance database you can't get the required performance data and therefore can not takeoff.
True, but not all engines are equal after they have been in service so you have to assume a limitation that covers these variations.
I have no problem with them sitting on the ground so I was just explaining the difficulty of scheduling a more tolerant machine.