There are, altogether, more XB360s and PS3s than Wiis. A cross-platform PS360 title has a larger install base to sell to than a Wii exclusive. Wii may well take >50% of the total market over the next years, but at this point in time it's not like PS2 versus GC where the PS2 market was several times the size of GC or XB, and GC+XB weren't sharing libraries in the same way XB360 and PS3 are.
The margin is slim, and it may not be long before the Wii matches the other consoles worldwide. It might take longer in the US, but that may just be because of lack of supply. But otherwise, true enough. Maybe the Wii will never become the next PS2, it just can't achieve that critical mass.
What Wii supporters claim, though, is that all those people who were going to buy a next-gen console when GTA4 came out, but didn't, really migrated to the Wii -- which would, sorta, make the Wii the new PS2, in that no matter the genre, you'll find a customer base for it. Again, I haven't seen any support for this, unless you somehow read this fact into the Wii's phenomenal sales. Again, I think we'll be able to have a better idea if this is true or not when Madden 2009 and Guitar Hero 4 come out, since those are 'casual' franchises where the PS2 does incredibly.
Personally, I think that the PS2 is still going to do very well. In general, I suspect the western publishers wish they had stuck a little longer with the PS2 (PS2 still gets a slow, but steady stream of games in Japan), though I admit I have no evidence to support this.