I think it's been hashed out enough already, but my general feeling from reading various books and magazine articles about the time period tracks with everything else here. Pennsy was extremely conservative in design, they focused on electrification, and they didn't look very far ahead into the future for alternatives. They were the Standard Railroad of the World after all, right?
When someone woke up and realized how far behind they were in the motive power department, they went for the duplex and just kept doubling down on it, tried a turbine or two, and then grabbed for diesel power of any kind. If I was a time traveling individual I would have the PRR motive department design a relatively conservative 4-8-4 in the 1934-1936 time period. Maybe 63"-70" drivers for a freight/mixed traffic version, and 80" for a passenger flyer. Or just a 70" driver version, and later advances in counterbalancing would allow it to hit higher speeds during postwar travel.
Reading about all the different modifications they made to the basic K4 makes for interesting read. Saturated to superheated, hand firing to stoker screw, and their later experiments with front end throttles, poppet valves, trailing truck boosters, and roller bearings. If they had run all their K4s locomotives through the works and added trailing truck boosters to compensate for their tall drivers and increased traffic loads, front end throttles and poppet valves for more efficient working, cast frames, and roller bearings throughout, they could have squeaked by without duplexes.
Of course my favorite PRR locomotive is the J1 which is just the C&O Texas with a Pennsy cab, so don't pay too much attention to me.
