Yeah...the thing with not being particularly quick is that, if you DO lose a step, you are suddenly skating in cement. I agree that a player who relies entirely on their speed is up the creek when their speed drops off, but guys like Subban (or the much greater Nik Lidstrom) are not one-dimensional speedsters like Russ Courtnall; they have a range of assets that they bring and will continue to make them effective even after the speed declines somewhat. (What we can say is that Subban will have to adjust his game; he will no longer be able to spin-a-rama his way out of jams and will have to simplify his game in just the way that the Therriens of the world have been calling upon him to do since he was a rookie). But if you're a guy whose quickness is merely "acceptable," and you lose that, then you are going to struggle to get yourself in a position where your other attributes can be deployed.
This is, I believe, the line of thinking that leads people like myself to be concerned about a drop-off in Weber. Weber is actually a very powerful skater once he gets his stride going. But when he loses his first step, that's when we're going to see the kind of struggling that even the mighty (and loathsome) Chara has succumbed to in recent years.
This is why I believe a Subban, barring injury, can continue to star in the NHL for another decade. I don't foresee the same for Weber.
That said, it's not really worth worrying about RIGHT NOW. It's a concern for down the road.