Casuals can and will lose interest.
The question is, how long will it take? In the case of the NES, they ended up buying the NES in droves, then fizzled out on the SNES (12m less units sold of the SNES than the NES).
IMO, the Wii's casual 'novelty' appeal will probably last for most of the generation. The Wii lifespan will probably be 2 generations. Wii one will do great, start to falter, and then Wii2 is launched, that re-invigorates the Wii franchises, but doesn't reach the predicessors total sales.
Even though some devs have said they will focus on the Wii more, I fail to see the big-name IPs that have switched, Yes, maybe alot of smaller useless Japanese franchises, but in the US and Europe, most franchises are still strongly PS3/360. Because of this, alot of middlecore's that got PS2s might not pick up Wiis when their favorite software titles aren't there. There are alot of people, as said, that grew up on Mario, but there are even more that grew up on mid-90s Final Fantasies, Halos, GTAs, Grand Turismos, Resident Evils, and the rest, that once they look into a new system, might not consider the Wii for a first tier system.
The emphasis of casuals has also brought in atrocious tier ratios for the Wii as well. Although there's a huge difference between PS3 tier ratios, and X360 ratios, the fact is, if the Wii is supposed to be the #1 system, the tier ratios just aren't there. They are mimicing the DS and not the PS2. The PS2 and Xbox had ratios of 10:1, and the GC even had 8.5:1. Right now, the Wii is just at 2:1 in Japan, and 3:1 in the US (not including Wii Sports). That's pretty bad.
Back from the dead, I'm afraid.
