I've owned both, and sold both, and I 100% agree - zero overlap how I actually used them.
- The 135L was shot at F2 or F2.8 for portraits and also used as an indoor sports lens - always shot at F2.
- The 70-200F4 was used at F4 to F8 on my 5DSR for tack sharp portraits with OCF
As I also owned the 70-200F2.8 II, I didn't get enough use from the F4 version, so I basically traded it in for the Sigma 24-105 F4 that I use for the 5DSR with Flash for Portraits. I also didn't use the 135L enough, just not quite my FL, and I just could not get it to work with the 5DSR unless I used a 1/640-1/1000 SS so I swapped it for the Tamron 85 1.8 VC because of the VC.
If the the rumored 135L II IS, indeed comes with IS, then I think you could almost argue there is some overlap - but in that case, I think the overlap will be with the 70-200 F2.8 I IS - not the F4 version.
You couldn't just use your 135L at f4 to f8 to take tack sharp portraits with OCF?
What do I know; I do difficult lit church photography and portrait type work; I have a 135, the 70-200f4 and an 85 1.8, and think I should toss all 3 and go Tamron 70-200 2.8 VC and Sigma's new 85 ART. Love my 135 but I think I'd love the 85 ART more, could do indoor shooting more effectively, and with a 70-200 2.8 believe I could get by very well without my 135 for everything else.