I rest my case for a new version of the 400 F5.6 ... it's coming in 2nd as the most popular lens.
New electronics. USM and IS ... latest glass coatings, perhaps a tweak here or there to the optics, a lick of new paint .. oh and the most important issue I think is a closer min focusing distance in line with the 300F4 or the 100-400 and it'll be a winner.
I'm stuck using a 70-200 2.8IS and at times a 2xIII, but wanting the "prime experience" with a clean, light, straight 400.
Not that keen even on the new 100-400, but may only get it for it's close focusing .. if it's the same as the old one at 1.8m. I don;t really need to lug around the extra stuff, as I really just want 400mm
Does sales have to slow down on a lens for Canon to think about a replacement ?
It still seems to sell well ... a friend of a friend only posted two days ago a pic of his new 400F5.6 sitting on the front seat of his car driving home from the shops after just purchasing it.
So for me it's a toss up of the closer focusing of the 300F4 or the reach of the 400F5.6 ... but the bigger issue is both lenses are so old they both are long overdue for a re-vamp ... and I want the new re-vamped models, not going to buy the older version no matter how good they still are, they are due for a new version of both.
Three years ago I was in the same situation: 100-400 or 5.6 400 or wait vor a mark ii version ... I bought the 5.6 400 because the price gone up (1500 Euro) but I had one dealer who sold it for about 1200 Euro. So far it was a reasonable decision because no mark ii version hit the market in the meantime.
Your points are very good: minimum focus distance and missing IS limit the 5.6 400 but on the other hand this sturdy, compact and well designed lens gives you at least a 1 or 2 stop advantage compared to a not so sturdy design.
The IQ is stellar - just atmospheric turbulence limits its sharpness/contrast/IQ moderately to severely depending on the weather conditions/local conditions. So the 100-400 will give you the same IQ in the field most times I think. And I like to have the ability to use f/11 @ 800mm with a 2x TC (mark i) which works very well with this "lens of the 1st hour".
You have the 2x TC so the 4.0 300 might be an option for you with a 7D ii which has AF on centerpoint with f/8.0 @ 600mm ... but: no easy decisions at all ...
Last point: Check IQ under field conditions between your 70-200 with 2x TC versus 5.6 400 if you have access to a 5.6 400 - perhaps the difference isn't that big and you do not need to lug around to much gear.