I've used the 70-200/2.8's since 1995, but only ever in the context of the 1.4x and 2.0x. That said I guess many pros saw/see the 70-200/2.8 as desirable even when used by itself.
I'm sure its performance was at least a little hampered by having to look good by itself while also working pretty well with the TC's.
I also thought its non-extending zoom was the best protection against sucking dust, moisture, and fungal spores inside the lens. That said, the extending lenses such as the 20-35, 17-35, 16-35, and 28-70 and 24-70's didn't seem to have problems per se along these lines.
For these reasons, the RF70-200/2.8 with no TC compatibility had no attraction whatsoever for me. It never even crossed my mind to get it. But, I also never questioned why anyone would.
I love the 100-500 and to me I treat it basically as a 70-200 that kind of has a 1.4x and/or 2.0x when I want them. I take the exact same kind of shots with it that I did with the 70-200/2.8 and tele-extenders, except possibly, the portraits from 100-200. And for those I have the 135/1.8 which easily does a better job than the 70-200/2.8s. (My fave EF was the 135/2.0.) So, now that Canon has a TC-capable 70-200/2.8 coming, I'm also not going to be interested in that, as I've already got everything I could use between the 135/1.8 and 100-500.
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Another huge problem I have though, is that I earnestly think the f/2.8 trinity concept is behind us. We needed it in the past, first and foremost, because our AF wouldn't even function fully without it! But in addition, without IS and without the sky-high noise-free ISO's we enjoy today, there was always a battle to get a proper exposure. We had to obey the reciprocal rule, and even 400 speed film was unacceptably grainy for much professional use. To be able to get even one extra stop from the zoom meant our keeper rate could double due to faster shutter, and we could get by with slower film some of the time.
A second issue is that shots were never really in focus, especially where the subject wasn't exactly centered, unless we laboriously selected AF points off-center. We couldn't reliably focus-recompose-shoot as few or no lenses had perfectly spherical "planes" of focus. A lens that was in focus at 3m for a centered subject might focus 3.5m away towards subjects at the 1/3 2/3 points. We'd always push the shutter speeds too low and camera movement or subject movement would cause motion blur as well. Since the shots were blurry and grainy, we couldn't use huge images. And in our small images, we needed f/2.8 bokeh to make the subject pop out of the background.
Now we can use ISO 4000 for many shots, and hand-hold 1/2 to 1/15 even at 50mm without a thought. Suddenly exposing by candlelight at f/4 is without worry. The AF absolutely nails the eyelashes every single time. Since the grain-free images are in perfect focus it's nothing to have your image the width of an entire monitor, and at that magnification size, even f/4 is enough to make the subject stand out from the background.
For these reasons, I think the f/2.8 trinity is actually a dead man walking. F/4, including the 100-500/4.5-7.1 (with the same bokeh-ful 70mm aperture of the 70-200/2.8) have long since been the tool the typical photog needs for the vast majority of their shots.
Note I still love bokeh as much as anyone and love the 135/1.8 and have owned the 50/1.2 and on EF, had the 24/1.4, 35/1.4, 50/1.0 and 1.2, 80/1.2 and 135/2. I get that. But for me this is more a special effect and I never sold an f/2.8 photo back when I was a working photog that I couldn't have sold had it had the DOF of f/4.
