Fair enough, I'd like to see an example of this. I think you have the 24-105/2.8 on order? You shoot indoor nighttime high school sports or similar?
I did preorder the 24-105/2.8. Concerts/events/sports, both indoors and out (typically at night with high school lighting). The 24-105/2.8 will pair perfectly with the 100-300/2.8 for me.
Once it shows up, perhaps you'd have time to shoot some shots with 50k at f/2.8, and 100k at f/4, and perhaps additionally 50k at f/4 but double shutter duration. That'd be I think a huge lesson to all of us of the utility of 24-105. That would really be exactly what Canon needs to sell this lens: new lens gets the shot, but the old one is either too noisy or too blurry. (Or simply not enough bokeh to make subject pop at modern typical presentation on big LCD monitors.)
And it would also wipe out my objection that "if f/4 doesn't do it for you, it's unlikely f/2.8 will suddenly be enough." If you can show that f/2.8 really looks so good that f/2 (f/1.4, f/1.2) simply isn't worth considering, and yet f/4 just has too much noise, motion blur, or DOF, you'll have totally disproved my theory.
I doubt your hypothesis can be disproven, because it's all a matter of judgement and personal taste. Does f/2.8 blur the background more than f/4? Objectively, yes. Is the background at f/4 not blurry enough? Depends on the shot and who's viewing it. Will f/2 or f/1.2 have a creamier background than f/2.8? Yes, but the tradeoff is the flexibility in framing you get with a zoom.
You can categorically state that FOR YOU, f/4 is just fine and there's no need for an f/2.8 trinity. But only for you.
Also to be clear I admit I was over-speaking when I said "the ONLY reason" and "NO-ONE needs" this. I'm sorry about that. I still stand behind my general argument that f/2.8 trinity is INCORRECTLY treated as the go-to no-brainer automatic decision any pro would make. You (or at least, indoor nighttime sports shooters) may be a special case. And yet this lens isn't being described as a narrow tool for indoor nighttime sports shooters but rather something a huge number of pros might consider.
There are lots of situations when one stop matters. Think basketball with a 15(16)-35/2.8 under the net. Light is not great, and the 1/500 s will be have motion blur but 1/1000 s will not. I don't shoot in those types of settings (not that close to the action), which is why I swapped my EF 16-35/2.8 II (<1% of my shots were wider than f/4) for the EF 16-35/4, then that for the RF 14-35/4. But for normal and longer focal lengths, I find f/2.8 better in many cases.
Specifically regarding the 24-105/2.8, the 70-200/2.8 is a great lens for portraits and I often use it in the 70-100mm range for that. I think the 24-105/2.8 will become an automatic no-brainer for pros because it enables wide shots, portraits, and low-light use without a lens change.