How many running backs are having career years post 35? Ovechkin hit his career highs for points in his early 20s. Lebron and Currey are great, but not as great as they were 5+ years ago. In various sports roles, you can make up for declining physical abilities with experience and intelligence. McDavid's game is based around speed, so he'll need to reinvent himself a little to continue winning Art Ross trophies.
No one thinks that McDavid's play is going to fall off a cliff. Just that it'll be hard for him to have career years going forward. And yes, his play will slow as he goes through his 30s.