If you use this site's data and align the three consoles by date of launch, the Wii would have 55% market share at this point (even after the effects of having the 360 be the only current-gen console on the market for a full year).
By contrast, the PS2 had only a 53% market share at a similar point (launch-aligned) despite having a full year's head start over its competitor (i.e. the inverse of the Wii's situation).
From this, I can safely conclude that the Wii can at least feasibly demonstrate the same long-term market dominance as the PS2, although there are many unaccounted variables.
But that's not directly addressing your question, which I take to be "how can the Wii sustain its incredible momentum long enough to pull a PS2"? I originally had a much longer post, which examined the DS' history and made comparisons, before picking the comparisons apart. But let's try this instead.
The mass gamer does not seem to care about graphics or online. By "mass gamer" I do not refer to the average forum-poster (although that holds true for some of us too), but to the masses as a whole. Nintendo simply needs to continue feeding their desires to sustain the momentum on that front, and I wouldn't deign to tell them how to do so.
But you're doubtless concerned with the more traditional gamer, the one who has voted with his dollars in favor of an HD console. For them, Nintendo needs only one thing to lure them in: more games.
They're seeing success by creating games targeting everyone, especially gamers willing to play something different. If they (or more likely, third-parties) start making games targeted more towards the traditional gamers' more narrow tastes, they will start to chip away at that block as well. Traditional gamers are, after all, gamers, and they go where the games go.
Some die-hards, of course, will refuse to budge, insisting on the HD consoles' better graphics and sheer power. But then, to this day some die-hards take the same attitude towards the DS. I would submit that that group is a miniscule minority. Many folks are entranced by the HD features, but they are even more entranced by good games made to appeal to them. If the Wii gets those, the gamers will inevitably follow.
*Insert short history of DS here*
I don't think this is too far-fetched: when the Wii was first unveiled, I recall all but the most stalwart gamer greatly anticipating what the Wii's new controls could do for traditional games, despite knowing that the graphics couldn't hold a candle to the HD systems. Many are disappointed with the system because that promise was not kept (*insert comment about needing to broaden horizons here*). But if it does, if we get those 1:1 sword-fights, those quality FPS games with MoH's controls, those RTS games, etc., I have no problem seeing most gamers changing their tune within a year.
