Where's the option for just 'no'?
The Wii has hardly stopped selling like you're suggesting. It's a little below the other consoles, yes, but with a price-cut and compelling software I see it reclaiming its first place, for at least a few weeks.
The definition of a 'limit' is a number which cannot be reached, ever. If you're suggesting that this 'limit' was always there from day 1, then you're wrong. Wii sold because it was cheap, new and had a fair amount of great SW. And it still does. The Wii isn't tending towards a limit. And, if I'm honest, these 'doom' threads are starting to get old.