The Playstation audience is not static. People are suggesting that because 100 million PS2s were sold, there are 100 million people waiting to buy the PS3. This is wrong for a number of reasons:
(1) Of those 100+ million sales, a significant portion were repurchases (say, 10%)
(2) Of the remaining 90% installed base (this is generous), let's say 1/3rd of those people are now too busy with their lives or are just no longer interested in gaming.
(3) A huge percentage bought the PS2 when it was under $200, which will be a long time for the PS3
(4) A significant percentage of that market could easily shift to the Wii, to the DS, or to the 360.
There aren't 100 million people waiting to buy the PS3. That's a ridiculous idea. That market shifts every single day. In this particular case, past success guarantees nothing for the future.
I can't imagine what PS3 and Wii sales will be like when it is all said and done, but I suspect neither console will end up with less than 50 million sold. I suspect that the Xbox 360 will sell about 40 million after price drops and before Microsoft replaces it (and pulls the 360 from the market in favor of its successor).
I will also suggest that the 'xobx' brand will have cost MS more than 5 billion when the 360 is retired. This means they will profit less than 2 billion between now and the 360's end of life. I guess you could consider this a 'duh' prediction.