UE4 might end up being the Wii-U's version of 3DS scratches and "I'm waiting for a revision". I expect a year or more of some people wilfully ignoring the excellent first and third party software on offer on Wii-U, neglecting to notice that its the best looking stuff available that isn't on a PC, and then come E3 2013 - some people will try and hail this as a reason to overlook the system altogether. Actually, if the demo impresses, they'll probably hail it as a reason to overlook it this week. Not everyone will be so closed minded. Its not like there's a UE4 supporting console releasing this fall to do any damage, and it remains to be seen what the cost of entry will be next year to one that *does* support it.
Truthfully, I think the telling thing will be when people continue to buy what will be perhaps technically inferior SKUs of software on 360/PS3 because that's where their friends lists are, or because they prefer that console and its controller or whatever... that will tell you how important this is and where Nintendo's priorities should really lie. They should have (and hopefully have) concentrated on making the console as appealing and affordable as it can possibly be - meaning complete overhaul of gameplay and services, while making it as strong as they can within those constraints. If they've planned it well, and execute it well, they'll have a hit on their hands no matter what anyone on here says or feels about it.