Of course everyone is entitled to their own opinion and, as CR Guy admitted, this post was based on a speculative conversation, not on any rumor, CR1 or otherwise, but, I just don't see Canon abandoning the 100-400 f4.5-5.6 zoom.
I agree with those who don't see the 70-300mm as a replacement for the 100-400mm. The extra 100mm is just too important for wildlife and other uses. (Especially since, on a 7D it's an effective 160mm difference, not 100mm).
These are two very different lenses and are not mutually exclusive. While an updated 400mm f5.6 prime could possibly substitute, it would be a poor substitute. The ability to zoom is just too important for wildlife and other uses. In addition, the combined cost of the two lenses would approach or exceed $3,000. Personally, I see the 100-400 zoom and the 300 f4 as a better combination for the money â€“ gaining speed and sharpness in exchange for a shorter focal length when needed, and then having the extra reach in the zoom when conditions permit.
The 100-400 mm fills a different niche than a 300mm zoom, because of its weight and size. Right now, the third-party lens makers aren't really competing in terms of quality, but if Canon leaves this niche unfilled, they will step up.
I've certainly questioned the whole strategy behind the 70-300mm. I now think Canon produced the lens planning for a very long life cycle, not expecting to recover their investment for several years, but feeling the need to have a high end, super sharp, heavy-duty, but relatively compact zoom in their lineup over the next decade that can handle new improvements in sensors. If you read their announcement you'll see Canon made much of the ability to use the lens in the midst of rain forest. Nobody is going to want to lug a 100-400 through a rain forest.