This is great news and exactly what I want, been waiting for this for years. I’m currently running a Sugino 46/30 with an Ultegra 11-34 on eTap (shouldn't work, but does), but old knees and big hills would benefit from a lower bottom gear and this will do it perfectly, without compromising the closer ratios for normal riding.
The 46/30 running an 11-40 (bottom 20”) will come very close to my old XT 22/32/44 touring set up (bottom 18”), so I can see this appealing to tourers as well. As for hydraulics and discs, having run them for a couple of years without any issues, I love them and wouldn’t go back, but then I'm unlikely to be touring far from civilisation (and it wouldn't concern me if I was).
I don't think I'd bother with the wheels, more than happy with my Hunt wheels, although the freewheel is too noisy and Shimano freewheels are always so quiet ...