I think that the current 400 f/5.6L fills a unique niche, "inexpensive lightweight prime supertelephoto with fast AF", that allows beginners to try working with a supertelephoto and (once techniques are learned) get really good photos. The quality:price ratio is highly favorable. The focal length is ideal for beginning bird photographers. I know that I wasn't ready to plunk down the money for an f/4 supertelephoto when I started, and I didn't know how I would do in shooting handheld. Certainly it is better to learn handheld shooting on this lens than on a 500 f/4 (particularly the version 1 at 4 kg). At the time I bought the 400 f/5.6L, it had by a good margin the best image quality in lenses under $2,000.00. I considered the 100-400L IS v.1 but since I had a 70-200 f/4L plus a 1.4x TC, I didn't feel that I needed another zoom. There are more choices now for the budget birder, many people will go for the Tamron 150-600 or save up for the 100-400L IS II. The next step up for me would be an f/4 supertelephoto.I agree, and if a 400F5.6 version II came out, you could expect much better performance than the 100-400 MkII. I think it would sell.