But for all the reasons you mentioned, this is a very versatile, remarkable lens that is easy to take along with several other lenses and another body.
Someone needs to come out with a current high quality 400mm f5.6 prime. I would switch systems for a high quality handheld bird lens.I think that there are a lot of people interested in a new version of the 400F5.6.... I would get it....
I love my 400mm f/5.6L... Is there anything this "lot" of people need from it that it doesn't have/do?! Weather sealing, sure, but asides from that I love it's relatively light weight and small size without IS/zoom/wider aperture - coupled with the 5D3 sensor's ISO handling this thing's even incredible when losing daylight!
I guess closer focusing would be nice but that'd maybe ruin the magical optical formula!
Cameras: 1DXII,5D III, 5D II. Lenses 24-70 2.8L II IS, 70-200 f4L IS, 70-200 f2.8L IS II, EF 400 5.6L, 300 2.8 IS II, Samyang 14 mm 2.8. Flashes: 600EX-RT X 2, ST-E3-RT, 580 EX II.
Plus lots of stuff that just didn't work for me