You are jumping through hoops.
In an animal evolving high core body temps, there would be two solutions. Individuals with gene mutations allowing efficient sperm production at higher temps would out compete an animal that was slowly evolving a way to get its testicles outside the body.
The bird’s solution makes sense.
You are assuming that the former method might achieve success faster than the latter one. You are also assuming competition as equally distributed. The evolutionary path of birds might have resulted in the heat-fighting method, which involves complex biochemcal changes, more successful than the descended testicles method.
In mammals with generally low frequency of reproduction as compared to birds, the physical descention method might have been more viable.
The variables are vast; and the success of each evolutionary path depends on those variables, and their timing of play.