They won't be able to brute force WiiU out of ports, end of story.
The U has an up to date GPU with 2011 level features, they would literally have to build a console 20x the power of WiiU which would bankrupt Sony and lose MS even more money from their gaming division.
PS4 / 720 will be at most 5x the power of WiiU imho, the U will be more than capable of running 'downports' (most next gen engines are so customizable they will be able to run on the Ipad 5)
It's misleading to say that because an engine can create content for an iPad 5, games made with said engine for PS4 or 720 will be scalable down to iPad 5 (or Wii U). It doesn't work like that.
Nor would the power envelope targeted by games need to be 20x what Wii U can provide in order to rule it out of a port.
The core determinant is what performance levels a game targets. If pubs decide Wii U has to be included, devs will have to work with that target in mind. If pubs decide it doesn't have to be included, it won't be. Devs will be able to make games with 'scalable' engines that can only be feasibly run on high end hardware, or make games that can run from the range of available hardware. It's up to their publisher and then their own ambitions.
I'll also note that Wii U is
already losing content because of performance difficulties. And I'm unconvinced that a relative shift in processing load from CPU to GPU with future games will make matters better here. Wii U's GPU is likely going to be outclassed by the others' GPUs by a greater factor than its CPU is by PS3/360. If devs/pubs decide to design with those power levels in mind, particularly in the realm of non-rendering work, it won't be good for Wii U.
In the end I come to the same conclusion as you - but for different reasons. What matters is publisher's decisions - but because it will inform the performance level devs target for their games from the start, not because Wii U could handle a down port anything devs might come up with targeting machines 3 or 4 or 5 times faster and just need a publisher OK to make it happen.