Thanks for posting that video, Corey.
Maybe "decline" might actually be considered in a different perspective: as maturation. Like Louis Armstrong, I think Monk's playing in later decades was less facile, but more idiosyncratic. They both became more inimitable later in their careers. Armstrong wasn't pulling off the same type of pyrotechnics from his 20's playing during the 50's, but his sense of nuance, to me, developed immensely.
There's this obscure trumpet player, I think his name is Wynton Marsalis, who said: "Pops could play one note, and it would be jazz." The same concept (despite the source) might be applied to Monk.