The 100-400 v2 seems like the current winner, if that's what you want. I still prefer the 400 prime. IS would be nice, as I don't like flash photography, but for me, weight and focus speed are biggest factors. It is perhaps once a month at most, more like 2-4x/year that I need to back up to take a photo or miss a photo because I'm too close. Despite this, I actually wish I had the 300/4 a lot of the time just for less weight. I am a field biologist and birder first, photographer second. Lugging around that s-o-b 400 prime all day, every day, 8-12 hours up and down mountains and through the mud and muck gets old. I leave it behind a lot due to being sick of carrying it, and then I miss some amazing photos, usually of dead rare species. I missed a Crescent-faced Antpitta perched in the open in perfect light for 20+ seconds as I was too sick of the camera to carry it that day.
While there is a big market for 1DX + 500/600/200-400, there is a bigger market for the 7DII + a ~$1000-4000 lens, and it boggles my mind why no one has made a great EF-S superzoom.
I have always wanted, and always will want, an EF-S 400/500/600 range f/4 or f/5.6 L quality prime. There is an entire world of crop-body bird photographers out there carrying around 2.5x as much lens weight as they are utilizing.
Weight really is the top factor for me, within lenses that have enough reach and image quality. I would pay $3000-4000 for a EF-S 500 or 600/5.6 with modern IS that weighed significantly less than the 100-400v2 or the 400/5.6.