I can hear distant echoes of the British climbers' refrain "but what's he ever done on grit?"
Surely it's not possible or useful to compare the two and we should expect that today's top mountain runner will be able to beat Billy's times. I'd certainly be surprised if he can't but that doesn't detract at all from Billy's status and achievements from a certain never to be recaptured time and circumstance, some might even say "golden age" - I don't know, I wasn't there. It's like trying to compare Mummery with Steck, Hannibal with Napoleon or Elvis with Strummer - impossible, pointless, to the detriment of both parties, and better left to lie