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.
5D3, 5D2, G5X, G16 | SY14/2.8, V20/3.5, 28/2.8 IS, Ʃ35/1.4, 50/1.8, 50/1.8 STM, Ʃ50/1.4 EX, 100/2.8L IS Macro, 16-35/4L IS, 24-105/4L IS, 70-200/2.8L IS II, 1.4x II, 2.0x III, T28-300 VC PZD, 70-300L IS, Ʃ150-600 OS HSM S