First, it won't deal a huge blow to sales or any blow to sales. Anyone who cares enough about Brawl to eagerly anticipate online matches in it will care enough to buy it anyway when it comes out without online matches. SSBM was the top-selling GCN game and SSBB will be the top-selling Wii game.
Second, sure he *wants* online multi, but the fact is that current internet connections are not fast enough to properly allow non-messed up twitch fighting games. A few have tried, like SF2 on XBLA, but the game system itself is slowed down and all the hardcore still complain furiously about how messed up it is because of lag. Most of the big-name fighting series are going to have online in the next iteration, and someone will get the combination of mad network programming combined with natural increase in internet speed right eventually, but it sure ain't going to be Nintendo, the worst company at online gaming in the history of the world.
So either
A) There won't be online because they can't get it right and they don't want to release a crappy mode
or
B) They'll put it in anyway and people will complain nonstop about how bad and laggy it is, even though it's as laggy as any game, but it matters way more in fighters.