I don't know if people still hold these beliefs, in tandem, but I do know people held them at one point. I want to explain why it should induce cognitive dissonance to hold these two beliefs.
1. The Wii U and PS3 are both averaging about the same number of sales currently. According to vgchartz, up until the middle of March - the Wii U has sold 457,519. The PS3 has sold 497,512. Last year the Wii U sold 3,674,351 and the PS3 3,547,908. If they maintain similar sales, and live similar lifetimes, both of which are very reasonable assumptions, then they should sell about the same for the rest of their respective lifetimes.*
2. By the bolded proposition, then if you believe the PS3 will sell 15.6 million to reach the Wii, then so shall the Wii U sell 15.6 million to reach 25 million lifetime sales, outselling the Gamecube.
3. The converse is also true. If you believe Wii U will reach 25 million, then you must also believe the PS3 will reach Wii sales. This is unless you think PS3 sales will drop off heavily, which they shouldn't since the PS3 is still getting new releases and has room for another price-cut, or that Wii U sales will improve greatly while PS3 sales will remain the same.
*The exception to this is if you believe the PS3 will exceed the sales of the Wii U or will outlive it. If you believe this, then please substantiate such a belief in this thread, if you want to.
< In my opinion, neither of these things will happen. Wii U won't reach 25 million, and PS3 won't reach Wii sales.>
