The 800 has only been out for 5 years. The 300, 400, 500, and 600 version IIs replaced lenses that were introduced twelve years earlier. I don't think there will be an update of the 800 terribly soon.
I'm not sure I agree. The 600 II really obviates the 800L - the 600 II + 1.4xIII gives 840mm f/5.6 that's lighter and cheaper, and still delivers better IQ. Same for the 600 II + 2x vs. 800L + 1.4x.
There are good reasons that Art Morris sold his 800L and now uses the 600 II instead, and I really can't imagine Canon is selling many (any?) 800's any more.
Add to that the fact that Nikon is launching an 800/5.6 with fluorite and some very impressive MTF curves.
These big lenses are tough and expensive to design - but Canon has shown no reluctance to pass those costs on to customers (and the price of the Nikon 800mm gives Canon plenty of headroom on price).
It may be a while before we see an 800 actually available, but I expect they'll at least announce its development (and likely avoid mentioning a release time frame, they don't need more egg on their faces!)