Even if we assume that the Wii is gonna be just slightly more powerful than PS3/360 and not as powerful as Sony and MS next consoles, even then, this is still not the same situation than with the Wii.
Most people going this route are missing the fact that this gen was the jump to HD, to active online interactions, to downloadable services, and a big etc.
IMO the Wii's shortcomings came much more from those things (it being SD, failing at the online aspect) than just a matter of graphics. Wii games on Dolphin look great, and i'll be damned if Mario Galaxy 1/2 isnt one of the best looking games this gen.
Unless we get some sort of huge step forward in terms of infrastructure for next gen consoles (wich I really doubt, if I had to guess i'd say we're gonna get better graphics and not much else), this is hardly going to be a similar situation to what happened this gen.
The real question here is what this means to third party support and Nintendo seems to be very worried and active upon fixing that issue, heavy push in their last conference and whatnot. Like I said this is still little info to move forward. For what we know the new Sony and MS consoles could be as far as to give the Wii-U a decent life cycle in the middle, or they could even not support UE4 until late in the cycle. We know nothing.