Collishaw was instrumental in holding the Italians at bay for some time, and even making inroads on their airforce and infrastructure with what were very much 2nd rate aircraft. The North African theater was allocated a few Hurricanes, Gloster Gladiators, a few antiquated Vickers Wellesley bombers and Bristol Bombay transports when the war started. He eventually received Wellington bombers, Baltimore bombers, Hurricanes and Kittyhawk fighters with which he did a lot of solid work against the Italians and the Germans during the Crusader Offensive. He commanded the DAF through Rommel's battles at Bir Hacheim and the defeat of Tobruk. Gen. Ritchie was sacked and Auchinlech took over. Following that battle the 8th Army and the DAF had retreated towards Egypt and were trying to pull themselves back together when Rommel launched the first battle of El Alamein. In combination the 8th Army and the DAF fought off the strong attacks, but Gen. Auchinlech had little time left, Churchill would sack him in August during a lull in the battle and replace him with Montgomery.
So, overall Collishaw outlived many of the ground commanders in the theater, but it was time for new blood. I have a suspicion, and that is all that it is, that Collishaw was not willing to be rotated out of his command. Rotation was a standard procedure, everyone got worn out on the same job if they stayed too long. Perhaps RAF HQ retired him instead of arguing (it was a very active and tense situation in July 1942) and Coningham took over the DAF.
Miles
BTW see my bio of him at :
http://www.constable.ca