There is a difference between "power at speed" and "power on a steep grade". You're not going to run 60mph up through Horseshoe Curve, so the slower-speed lugging capability is what matters. The best engine for the hill (at least in today's standards of not delaying NS freights) is the engine that produces the most amount of horsepower at 25-30mph. The C&O T1/PRR J1 are basically a Berkshire on steroids. More tractive effort/more horsepower = more ability to move equivalent tonnage ratings at higher speed = faster ascent up a 1.8% grade with that train. Same basic analogy with a C&O Allegheny. You're well below the speed for the horsepower peak for an Allegheny, but there's so much power to start with that it doesn't matter for that amount of tonnage.
As far as the NKP Berks outlasting the other engines, this is considerably murkier water. Without a doubt, the NKP Berks were some of the best utilized steam engines. The Nickel Plate found what the engines were best at, and--unlike many railroads--used them in that service to their maximum potential. They filled a void, and the Berkshire's ability to run at high speed with freight made them perfect for what the railroad needed. This wasn't a case of shoehorning a round peg into a square hole, which is what a lot of railroads ended up doing. The Berks did EXACTLY what they were purchased for. With the perfect engine for the task, it made sense for the Nickel Plate to stick with steam until the economics for going to diesel were desirable.
Needless to say, conditions on the C&O weren't exactly the same. Steeper grades made the necessity for a Berkshire on steroids (aka T1) desirable. You get more power available with maybe a little loss on the top end speed. Good engines, but not good enough to starve off the diesels. Same thing goes for the C&O 2700s, which were essentially identical to the NKP Berks. C&O went for diesels fairly early on, and when they could get enough of them by the mid 50s, steam was on its way out.
Now the Allegheny is enough for a novel. Long story short, what the C&O needed was a big engine that provided a ton of power at slow speeds for getting heavy coal trains over the mountains. That's not what they got. In N&W terms, they needed a Y6 and got an A. The Allegheny really fits in with the Challengers and N&W As, not with a slow-speed lugging monster like a Y6. Of course, the C&O already had the lots-of-horsepower-at-high-speed merchandise freight engine with the T1. Herein lies the problem. It's not that the Allegheny was a bad design per se, but it was not needed by the C&O (and even less by the Virginian).
So instead of using the Allegheny where it could run at high speed (like a Challenger or an A), the C&O instead used them in pusher service over the mountains where the engines weren't being used anywhere near their optimum capabilities. 10-15mph is way too slow to get to the horsepower peak on that engine. Needless to say, the C&O wasn't getting its money's worth out of the engines and dieselization was a no-brainer. On the right railroad, the engines could have succeeded. The C&O and Virginian weren't those railroads.
Kevin
