On one hand I can definitely see this being Sakurai's last Smash game, if only because it's hard to imagine this whole process happening all over again. But there's no real evidence to support that, and we have no idea if he'll be tired of the franchise after Brawl. I think there's little doubt that there will be a new Smash game for each future gen, because it's just such an incredible money-maker franchise for Nintendo.
Either way, with or without Sakurai, I think the next one will be much more conservative. It seems like Brawl is the big overhaul of the formula, and from here on out I don't think we'll see anything nearly as exciting. Most likely the next game will just be Brawl 2.0. A game like this will always do fine with just better graphics, more characters, better features, etc. As long as they stick to that, I don't think they really need Sakurai as long as the dev team has the same basic philosophy.
Didn't that original article by Sakurai said that Miyamoto told him that if he didn't agree to do Brawl, they would basically just revamp Melee to have online play?