For those that want a 200-500 or xxx-600 or whatever instead, what if the bodies have f/8 AF points and you can use a 1.4x TC with the new 100-400? The Tamron really needs to be stopped down to f/8 at the 600mm end anyway, so this lens would be about the same as that one, just in a smaller size with more flexibility.
That sounds like a good deal to me.
At twice the price ?
Yes, if it's better optically, has better IS, has better handling, has better AF, and is smaller and lighter.
If I count correctly, that is six ifs.
I do hope it is released and is a big step up optically. For my uses, when my 100-400mm dies I will probably go with the big Tammy. I can use the extra reach, I often need to just identify distant birds.
I don't make much money from avian photography. So I guess investing 2x in a lens with only a small IQ improvement would be an example of the law of diminishing returns.
One other point I would make in Canons favor is that my 100-400 has been through hell and back and yet still works great. Hopefully the same would be true for the new iteration. We will not know about the durability of the Tammy in the field for a while.