Even the most successful R-rated superhero movies (Deadpool and its sequel) didn't do as well as movies like Venom. Now, I will be the first person to say that there are properties that can leverage an R rating (such as the aforementioned Deadpool and Joker) and do great box office. I will also candidly admit that this can be a chicken-and-egg issue; after all, the biggest movies are going to be PG-13* because they need to appeal to the biggest audience, which becomes a partially self-fulfilling prophecy.
Indeed, you anticipate my response. Given the weakness of the R rating, that's it's just 17 or an adult with you, it's hard to see it as any kind of real barrier. Especially as it's obviously plenty of people do take younger people to R-rated movies. I mean hell, I was in Santa Monica when American Pie came out, and I was old enough, but my little sister wasn't (14) but was desperate to see it, so I took her (after running it by my mum) - and she was by no means the youngest person in theatre (which did shock me a little). In the UK that was a 15, which is a hard rating like NC17 is in the US. Also, my understanding, which may be outdated because I last really looked at this in like 2004 was that in the UK 18-rated movies do better, relatively, than R-rated ones in the US - even though some Rs in the US are 15s here (not 18) and we have hard ratings, not soft.
(I notice you mention Venom - it had the same rating in the UK as Deadpool - 15 - and made about half as much money here as Deadpool did, so that does indicate the impact of the ratings in the US. This supports your argument, but also shows how culturally unique to the US it is.)
It seems like TV ratings are different and possibly entirely optional for streaming-only movies and shows so maybe this will matter less as time goes on, especially if cinemas in the US continue to decline. Alternatively the increasing relative importance of worldwide box office (compared to the "who cares?!" attitudes of a few decades ago) and the fact that censors are harsher in much of the world might maintain the focus on PG13.
It's fairly well-known that having an "R" rating is a massive disadvantage at the box office. For example, of the top 50 (FIFTY!) top grossing movies, how many do you think are rated R?
If we actually look at the top 50, we do see that easily 85-90% of them are from long after PG13 or bust became not just received wisdom, but a law of Hollywood, that if you didn't follow it, you attracted some degree of disapprobation. There's what, 3 movies in there from before 2000? And the vast majority are post-2010.
Joker is interesting because it's the only movie in that top 50 which seems like a serious piece of film-making (despite being staggeringly derivative), with the possible exception of Titanic and Barbie, or if we're feeling incredibly generous, Jurassic Park and the Avatar movies.
*Why not just PG? For complicated marketing reasons, "PG" and, of course, "G" movies are often viewed by the desirable teen demographic as "kids' movies," and that's considered a negative for a tentpole movie- outside of animated movies.
I'm aware. I'm confident Marvel puts easily-cut stuff in to ensure a PG13 rating rather than PG (easily cut so more stringent ratings boards outside the US won't have a problem with them).
To circle back to my point re: R-ratings and audience size, my point is that a lot of movies, even if they were PG13, are just never going to be huge top 50 entry types, hence "leveraging an R-rating" as you put it. More serious movies will not be making MCU/Pixar/Disney money in the shorter term so I don't think they're limiting their success much by going for R unless they have a profoundly family-friendly conceit.
(NB I never suggested The Marvels should be R - that was Henadic!)