While I agree that Cafe needs to be more than a third more powerfull than a PS3/Xbox360 there are 2 things that you need to consider.
1-Resolution. Nintendo chosed to launch a console without HD while PS3/Xbox360 used it as a main selling point. This meant that a multiplatform game needed a different engine on the Wii, another set of textures, etc. plus another control scheme. This time while PS4/Xbox720 may pursue the true 1080p at 60fps, developers will be able to use the same engine and textures on Cafe, but at 720p.
2-Development costs. As SaviorX points if Sony and Microsoft launch new consoles with a leap in performance comparable to the one from PS2 to PS3 that will lead to far too high costs, forcing them to look for as many ways to increase income as possible. Since compays refuse to launch most of its games on PC due to piracy, the other ways to find this income is lauching in as many platforms as possible, that means launching them on Cafe, or increase the price of the games. And knowing them they will probably do both.
As I (think I did anyway) pointed out in my previous post (moments ago), graphics aren't the only thing to a game engine. There's numbers of objects to track at once, physics, particles, players online, sizes of stages, framerate, and all sorts of other things. Even if the Wii magically output in High Def, it's CPU and GPU were still just "supercharged" GameCube parts. It still wouldn't have been able to handle the Unreal engine.
As such, if Cafe is truly just a dismal one-third more powerful than a PS3, then it might as well be little more than a "supercharged" PS3 or Xbox360. Sure, it'll be able to handle HD graphical output, but it's won't be able to handle the vastly more complex engines built to optimize on the PS4 or Xbox720.
For the other thing, ever since the N64 and GameCube, 3rd party companies have had to weigh the amount of profit they'd get from a Nintendo system compared to development costs. Sure, overall, they'd probably make more money porting said game to every system, but developers found on the GameCube that, even though it was easily in the same league as the PS2 and Xbox, games didn't sell on there. Many yanked GameCube support and simply stuck with PS2 and Xbox. Why even bother putting some games on the Nintendo system when they pretty much always sell vastly lower numbers on there compared to Sony or Microsoft's machines? Essentially, despite the viability of cross-platforms to make a profit, overall, many companies saw it as financially unfeasible to port their multiplatform game to the GameCube. They didn't even see the point.
It'd be different if 3rd party companies had any respectable success on Nintendo systems, but since the N64, regardless of quality, they generally haven't had success on Nintendo systems, because by and large, Nintendo fans don't support 3rd party games. They'd apparently rather have Wii Music or more lame-ass crap with Mario in it than 3rd party software. And sadly, it doesn't seem to matter if the 3rd party stuff is high quality or if the Nintendo stuff is basically shovelware. Wii Music still grossly outsold Bully and Dead Space Extraction.