I'm really surprised by the 4 to 1 out selling of the 70-300. The 70-300 is vastly superior to the 100-400 from 70-300mm. While no, it's not a great birding lens, it's extra 2 stops of IS and extra IQ are above and beyond the 100-400. As far as the method by which it zooms, is a complete non-issue.
I am surprised too.
The 300mm on crop 1.6 (for example 7D) is still 480mm. That is more than 300 or 400mm on FF.
Of course light conditions must be good - so there is a little limitation
I'm not so surprised considering most people probably use the 100-400 on a crop, that gives you 640 mm equivalent Add to this the fact the 100-400 is still the best way to get to 400 mm with IS on a budget, and that it 'collapses' to a very reasonable size as mentioned earlier, still make this lens a winner.
If the IS was equal, and the IQ was equal, I'd agree. But the IS is 2 stops better, which I personally find incredibly important on something as long as 300-400mm. Yes, it's 400mm, but it's not that
much longer than 300mm, and it is considerbly larger and heavier than the 70-300.
Then again, the vast majority of people buying camera gear are quite ignorant of what they're buying, so perhaps it shouldn't surprise me that much that people buy the longest thing they could afford.