Comparing the "three" - I suspect with the technology approximately equal, those that own the V1 will keep it, and the new buyers coming up into the Canon box will buy the V2 ... once the V1 runs out of stock. So, my guess is Canon figured this out already and built V2 for the new buyer market, not the upgrade market. The weatherseal is one benefit over Siggie for sure ... especially those of us that primarily shoot outdoors.
Those that like/buy Siggies will probably still buy Siggies...
Those that know technology better than me (HA, most of you) say it's as good as glass can get when building a 24-105 zoom range. So, maybe we're at that point, and anyone needs better glass can go with 16-35 and a 70-200 - filling that void with a couple prime lenses if necessary. Disappointing, that the new 24-105 lacks enough improvement, but we shot without it all these years, so will just continue ... there's never a true all-purpose mulit-purpose lens anyway ... most are for specific settings - so that didn't change at all.