I don't think they're commercially non-viable, as long as they're cheaper than tablets and possess longer battery life.
I agree technology can still be around even if it is "out of date," such as calculators.
It is unfortunate that e-ink is advancing much more slowly than LCD screens, where we see many types of improvements coming to market, though. I believe eventually there will be either Pixel Qi-ish LCD screens or e-paper with high saturation color (not enough people are going to want to pay $100+ for something that looks like newsprint), but not in the near future. So far the most impressive color e-paper (not e-ink) has been Fujitsu's Flepia cholosteric LCD screen (2011 prototype):
https://www.youtube.com/watch?v=0VmCrblDMlc.