It's not like athleticism and skill are used separately when playing, it's a continuum in which every player finds themselves relying on both.
So even if post injury he's still athletic relative to the rest of the league, it would still be a big change for him personally. If an injury took it down to 38 instantly, he'd still be very athletic, but that's now 6 inches off of his layups, jump shots, fadeaways etc that he has to adjust for in relatively speaking, what is a short amount of time. LeBron in his prime had a 44 inch vertical for example. People underestimate how important athleticism is in regards to muscle memory. When reading people's projections about the 2020-2021 Nets, I've seen quite a lot of people say that KD's achilles rupture will have little effect on KD, with some people even saying 'It will have no effect at all' because his game relies mainly on skill, not athleticism, and that Cousins and KD are similar in height and play style.