I am sure Canon can do it. I think it is more of a marketing decision. Obviously they don't want to introduce a f4 IS to eat up the fat margin f2.8 IS is enjoying now.
I really liked my 70-200 f/4L. The size/weight/image quality are stellar. I sold it to afford the f/2.8L IS. I'm glad I bought the 2.8 IS but I really wish I had kept the f/4 for those times when I don't want to lug around the 2.8 and don't need that extra stop or IS (which is actually more often than I give my my shooting style credit for).
If I did it over, I'd have kept both. But now, saving up the obsession fund I'm not likely to repurchase the f/4 - that is unless my wife starts shooting and doesn't want to lug around the IS .
I have the older 80-200/2.8L. I believe it weighs in about the same as the 70-200/2.8, and I really don't find the weight of this lens to be a problem.
With the tripod collar, it's perfectly balanced on a tripod, and that collar also makes an excellent place to support the lens with your left hand when handholding. When shooting stuff like soccer, the lens is up to my eye for long periods of time, and I don't find it that difficult. Oh, and I ain't exactly a big guy.
Shoot with the f/2.8 for a while and then put on the little f/4. It feels like a dream compaired to the big f/2.8! I owned the f/2.8 for a short while along with my f/4. Unfortunately, the expense of the 2.8 was a little much and I had to sell it. The one thing I noticed was just how perfect that little f/4 is compared to the big behemoth! I will eventually get the 2.8 again and keep BOTH lenses.
I just replaced my 70-200/2.8L non-IS with an IS. For light weight/low light, I opt to use my 135/2L. I have to move my feet a bit more, but the result is well worth it.
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