Heya,
Depends on what you shoot.
I have the Tamron 150-600, 200 F2.8L and 2.0x TC that I use. Gives me a 1200mm lens for moon and stuff (fun). Gives me a 400 F5.6 prime that is fast for birding from my kayak. Or just a fast 200 F2.8 for portrait or sports, etc. I shoot 1D series, 5D series, APS-C, etc. Everything really.
I don't feel the need to get the 100-400 MKII for $2k at all.
While it may be fast, sharp and amazing, I don't think I'd care about it as a zoom at 400mm for $2k. Frankly, if spending $2k on a telephoto, I want it to be fast, like F4 or F2.8. So for me, in a similar boat as you, when I think of trading in my telephotos, I think more about a 300 F2.8 non-IS than I do about a zoom with F5.6.
Ultimately the only zoom I'd trade in for right now, is a Sigma 120-300 F2.8 OS (non-sport). Around $2k as well. 300 F2.8, sharp and fast. Can take a TC and be 600 F5.6 and is still quite sharp and fast. To me that is more interesting.
Just a thought. And my perspective is that of wildlife, birding, in dark swamps and at the ocean, so I have a different take on needs & purposes and I handhold a ton, but I also use mounts & flashes.
If your primary use of the telephoto is handheld, in good light, and you don't need maximum reach, then the 100-400 II is an awesome lens for you.
Looking at your Flickr, you shoot a lot of aviation, I would probably trade in the Tamron for something with superior panning stabilization. If you're doing handheld aviation and trying to go for low shutter panning shots and stuff, the 100-400 MKII makes a ton of sense over a Tamron 600mm. The 400 MKII completely replaces the old 400 F5.6 prime (great lens, but it has finally been beaten, took a long time!). So if you get the 100-400 MKII, sure, replace both lenses. Essentially pays for most of the 400 MKII with a little out of pocket to finish off the purchase.
Very best,