I stopped being a Nintendo fanboy since the Gamecube era as that is when I went multiplatform and I haven't looked back since, owning every gaming system since then as it is the best way to go.
I think that Wii U would be able to hande PS4/XBOX 720 games if the developers take the time to make downgraded ports. I guess the "real" next-gen systems would be kind of like playing your games in high settings on the pc, with the Wii U version being the medium settings version, or maybe even closer to low settings in some situations. People would still get the same gaming experience and fully enjoy the games, just with a different graphical fidelity.
So imo, I think it's mostly a case of whether or not developers want to put in the effort or just skip the system. Looking at the Wii U launch it looks like support is good so far, developers just need to grasp how the system works. I really enjoy my Wii U right now and hope it gets as much support as possible.
Also, I think people will be thoroughly disappointed with next-gen games, as I think next-gen is already here, on the PC. When I see what is possible on an SLI equipped system (and I know it might not be as optimized as it can be on a console) , I honestly can't see how much better consoles can push better graphics, especially because of the price issue (500-600$ max price). I know next-gen engines will help a bit, but I feel sorry for people who think we haven't seen anything yet, because the PC master race has seen it all already!