I guarantee you that if Canon ever does release an updated 100-400, it will NOT be anywhere near f/2.8 nor will it even be a constant f-number. That is why the 70-200/2.8L IS is such a coveted lens--it is about as long as one can make a telephoto zoom whilst having a constant wide aperture. A 100-400 zoom will necessarily be something like f/4-5.6 unless you are willing to make the front element huge (or resort to DO). But that defeats the whole purpose of such a zoom and it's not going to be in the $1500 range anymore.


The 70-200/2.8L IS and the 100-400/4.5-5.6L IS are entirely, entirely different lenses for different photographic purposes, and not primarily because of the difference in focal length range. Since you are already happy with the design constraints (if not image quality) of 70-300/4-5.6 IS, you should simply go to 100-400 now because I don't see Canon updating this design any time soon. It will remain push-pull (an IF design would make the barrel prohibitively large and heavy for handholding), and it won't go much wider than it currently is, without becoming astronomically expensive. So I wonder what kind of update is desired...maybe more fluorite/UD elements for better CA control? Or...? Playing the waiting game means you could go another 5-6 years before Canon revisits this lens.


One last thing I would like to point out...based on your existing stated lineup you are missing out on shooting very fast apertures at the longer end, so I take it you're not doing a lot of portraiture or available light photography. That is the biggest reason to get the 70-200/2.8L IS, but you could be served quite well with any wide prime, 50/1.4, 85/1.8, 100/2.8 macro, 135/2L, or 200/2.8L II, each of which costs significantly less than the zoom and is sharper too.