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, Sony α6000 | SY14 f/2.8, Ʃ20 f/1.8, 24 f/2.8, 35 f/2, Ʃ35 f/1.4A, 50 f/1.8 I, Ʃ50 f/1.4 EX, 100L Macro, 17-40L, 24-105L, 70-200 f/2.8L IS II, 1.4x II, 70-300L, 100-400L | E-mount: SY12 f/2, Ʃ19 & 30 f/2.8 EX DN, 16-70 ZA OSS, 55-210 OSS, Metabones SB | FT-QL, AE-1P | FD(n) & FL lenses.