Hmm, I couldn't care less about the 70 mm position to be honest. I rather have a somewhat limited zoom range in favor of image quality.
That said, buy the Canon 100-400 while you can, it's (relatively) cheap - a MkII will definitely set you back more. I like the push/pull design for motorsports especially, being able to chase the subject as it rapidly comes closer.
I must say though that I've used my 100-400 less since I've paired the 70-200 F2.8 IS MkII to a 1.4 TC. The latter offers F/4 and better IQ throughout the range (98-280mm).
Still, benefit of the 100-400: reach, compactness (at 100 mm) and light weight. The 70-200 + TC is a lot heavier, something that counts on a long day of shooting. I wonder what the added gearing of a rotary zoom 100-400 would add to the weight...
5D3, 5D2, Sony α6000, G16 | EF: SY14/2.8, V20/3.5, 24/2.8, 35/2, Ʃ35/1.4, 50/1.8, Ʃ50/1.4 EX, 100L Macro, 17-40L, 24-105L, 70-200/2.8L IS II, 1.4x II, 2.0x III, 70-300L, T28-300 Di VC PZD | E: SY12/2, 35/1.8 OSS, 16-70 ZA OSS, 55-210 OSS, SY300/6.3 ED UMC CS, Metabones SB | Vintage FD & FL glass.