One of the most fun character pairings in Avengers: Infinity War was the entertaining clash of egos between Tony Stark and Doctor Strange. These two characters are both brilliant and arrogant, not unlike another role Robert Downey Jr. and Benedict Cumberbatch have both played: Sherlock Holmes. Robert Downey Jr. played the sleuth in the Sherlock Holmes movies and Benedict Cumberbatch in the Sherlock TV series. So you might think that Infinity War would take advantage of that fact and put a Sherlock joke in the film, but there wasn't one. Speaking on the commentary track on the new Infinity War Blu-ray, screenwriter Christopher McFeely and director Joe Russo explained why there was no Sherlock joke, saying:
Christopher McFeely: It's a meta joke, that requires you to be a fan of other movies.
Joe Russo: It's also a very obvious joke. Not that we can't be accused of having made obvious jokes before...
So basically, there is some fruit that hangs way too low to pick, and that was the case with including a Sherlock joke in Avengers: Infinity War. It might have been so obvious that it wouldn't even be funny, instead coming off a bit cringy. Even though Joe Russo says that they aren't above using obvious jokes, there is also the point that screenwriter Christopher McFeely cites and that's that such a joke is too meta. The Marvel Cinematic Universe films may reference each other and broad pop culture things, but making a joke about both actors having portrayed Sherlock Holmes might have been a little too specific for the majority of audiences to pick up on.
While many of the people reading this probably know that Robert Downey Jr. and Benedict Cumberbatch have both played Sherlock Holmes, not everyone would. The Sherlock Holmes films starring Robert Downey Jr. made good money and people saw them, but the last one was in 2011. And although critically acclaimed, not everyone has seen the BBC Sherlock TV series. Thus a Sherlock joke would likely have only appealed to a portion of the audience, and Marvel movies are meant to be inclusive. Requiring you to be a fan of these other properties is not its m.o.
I could totally have seen some sort of reference to "It's elementary" or "No shit, Sherlock" being thrown in for the astute observer who would pick up on it, and it might have been funny, but it's probably for the best that such a joke wasn't included. The reasoning Christopher McFeely and Joe Russo give is sound logic. A meta joke referencing the actor's other roles as Sherlock is reminiscent of Deadpool 2's "Pump the hate brakes, Thanos." In a fourth-wall-breaking film like that, we expect those kinds of meta references to other things. It's part of the experience, but it's a tricky balance to do that right in an MCU film and not take audiences out of the movie, even if it is a funny joke.
It is unclear exactly what the future holds for Benedict Cumberbatch's Sherlock, but Sherlock Holmes 3, starring Robert Downey Jr, is happening and is scheduled for release on Christmas Day 2020. If you want to see these two Sherlocks bantering back and forth in superhero attire, you can check out Avengers: Infinity War on Digital HD now and on 4K Ultra HD, Blu-ray and DVD August 14.