People claiming that something won't work is not a good heuristic for finding things that will work. It may be the case that people say that many ideas that turn out to work won't, but they are overshadowed by all of the ideas that people say won't work that don't.

True: People also said the CueCat wouldn't work. So sometimes the naysayers do know what they're taking about.

Perhaps I'm missing something, but that seems to have resurfaced (in a sense) with QR codes.

The big objections to CueCat, as I remember them, were that people would never install the hardware (and if they tried they'd need tech support), and that dragging adverts over to the desktop computer would be too much hassle.

QR code readers are a software (not hardware) install, and they install onto your phone (not PC), which is quite naturally close at hand when your reading off paper.

I'm not saying QR codes will proceed to establish themselves long-term. I am saying that if they don't, that will tell us more about the potential ecosystem for this sort of thing than CueCat's failure did, simply because major usability frictions inherent to the earlier technology are now out of the equation.

Agreed. I would add that, given you've found other ways to validate your idea, nothing inspires you quite as much to deliver as the opportunity to prove a naysayer wrong :P