McDavid probably has many steps to lose before he becomes even nhl average at speed. The thing that puts McDavids speed and agility over everyone is his puck handling/skills, and decision making at those speeds and elusive skating. Ppl talking about his IQ not being strong are full of it. I don't think he's going to lose high end decision making or IQ just bc he's older or slower. There's players in their mid 30s that are nearly ppg players (Pavelski, wheeler for instance), and I think McDavid's physical abilities, along with finish, and offensive IQ ahead of both at a similar age. Injuries of course plays a big part of every players longevity. I also beleive he's the type to not rest on his laurels and improve his game and id say he already aha.
Yeah I see very little chance of him falling off a cliff without a major injury being the reason. Anyone who thinks McDavid doesn't think the game at the level of Sid is obviously not watching the dude work. Sure his speed allows him to just outpace everyone on the ice but even when he slows the game down he can pick apart defenders.