I can almost guarantee he will never run anywhere near a 3:42 mile. He’s too big and tall. Plus, fast twitch sprinter types have been scientifically shown to really run out of stamina quickly compared to actual distance types due to a myriad of factors including the types of muscle fibers they have. 400/800 types begin to see diminishing returns quick at the mile distance. There is such a thing as being too fast to run too well in distance events. In Cooper’s case he is one of the guys who can stretch it out reasonably to about a mile, but runs out of stamina quickly. That’s why as of now he is a 3:46/4:06 1500/mile runner even though he is a 1:42 800 guy. He can run an okay mile, but not world class because it’s too long for him. Wilson Kipketer could barely run under a 4 minute mile. Check out a lot of the American 800 greats: Symmonds, Hoppel, Brazier, etc. They usually max out to around 3:35-3:38. The true 800 types have physiologies that work against them being better than average milers.
Now there may be a greater than a snowball’s chance in hell that Cooper runs a 1:39. He’s basically the white man’s answer to Rudisha. Remember Rudisha ran his 800WR leading start to finish in an Olympic final with no pacers. I can see a case where Cooper has more ideal conditions and help to break 1:40. Cooper is almost certainly a future low 45 sec 400 guy, which gives him the same speed advantage as Rudisha while also having the advantage of better pacing conditions, youth and room for improvement, and access to more sophisticated training and racing technology. It will be hard, but a 1:39 wouldn’t absolutely shock me.