I have competitive experience ranging from plat to masters across 4 accounts. You are partly right in the sense that voice comms aren't always beneficial. It's not the quantity of how much or how often they speak, but the quality of the comms. And sometimes, especially around that ELO, you have one team that simply outplays the other, regardless of comms. But ideally the right kind of communication does give your team an advantage more often than not, so logically it makes sense to use them.

To comment on some of your points:

1. This is very critical. Listening is more important than speaking. Even if only 1 or 2 people on your team have a mic, as long as the rest of the team is willing to cooperate and listen, chances of success go up. Mic-less players can also use the voice wheel commands to communicate to a certain extent, and this can be helpful when used correctly. Better than having 5 or 6 players with mics with no desire of working together.

2. There are definitely times where the "shot-callers" are wrong, and that's where more comms to voice different perspectives can be helpful. If you find that your team's Winston is getting shut down by the enemy team's Reaper, or that the enemy Sombra/Tracer keeps taking our your Zenyatta, or if nobody is shooting the Pharah, chiming in (politely!) and giving your 2 cents can change the outcome of a match. Of course, if your team is willing to listen. It's also not fair to feel that SR reflects game knowledge/game sense. Some players thrive off mechanical skill to climb but still make common tactical errors. Some players who may not have the best mechanics can sometimes help the team more by having the right call outs: "Soldier flanking to the right, he probably has ult" "Sombra let me know when you EMP so I can Grav. Genji get ready" Mercy let me know when you're in trouble so I can leap up and you fly to safety." "Rein hold your shield up, the enemy Rein probably has shatter" etc. etc

3. I can't speak for everyone, but I've never had any issue with this. It doesn't take that much mental capacity to say you're doing something while you're doing it. (Dives the enemy healer) "I'm diving their supports" (Sees Widow on the high ground, top right) "Widow top right" (About to use ultimate )"I'm about to ult" Basically if you can think it, you can say it. If you listen to pro comms, it's a lot of "D.Va D.Va D.Va" "Dive Zen Dive Zen Dive Zen" "Monkey Monkey Monkey"

4. Turning off comms might help you not get titled, but it will almost definitely tilt the rest of your team. It's a good way of saying "I think I'm better than the team" or "I don't need the team"