Swapping out one Defender for another makes perfect sense, especially since they're Earth's first line of defense in the movie.If you're already stressed about Avengers: Infinity War because there's just so damn many characters, you're not alone, and we're here to help.
But there's a deeper reason behind that choice: Hulk, Doctor Strange, and Silver Surfer are often contemporaries in the comics as a part of the super team the Defenders (not to be confused with the Marvel television series by the same name).
Since Silver Surfer is part of the Fantastic Four movie universe, Marvel doesn't have the rights to use him, so they went with the Green Goliath instead. Of course, the comics use Silver Surfer instead of Hulk, since the former Herald of Galactus was one of Marvel's premiere cosmic heroes at the time. While it's a great way to showcase Thanos' indomitable might early in the film, it's also taken right from the original Infinity Gauntlet comics. One of the movie's most striking visuals is Hulk getting teleported to Earth to warn Doctor Strange and the rest of the MCU's heroes that Thanos is on his way - and much more powerful than anyone could have suspected. With Cap demonstrating peerless heroism and confidence, it's hard not to imagine even the all-powerful Thanos feeling a sliver of doubt. Considering that one of Thanos' defining characteristics is his subconscious desire to lose, the scene might be even more meaningful than that. Like in the comics, Cap can't actually win, but his unhesitating willingness to keep fighting briefly impresses Thanos. It's a character-defining moment for Steve Rogers, and a moment that's adapted into Infinity War.
But there's one moment in the comic series that absolutely had to be adapted: Captain America facing down Thanos after watching him kill the strongest heroes in the Marvel Universe without breaking a sweat. Some are just too mired in Marvel continuity to make sense showing them on the big screen, like the cosmic deities of the universe teaming up to kill Thanos in a massive battle. Some of them have to do with rights issues, like the absence of the Silver Surfer or Doctor Doom. Plenty of moments in the Infinity Gauntlet comics weren't adapted for Infinity War. No wonder MODOK develops a massive crush on her. Scheduled for a 2019 release date, Captain Marvel stars Brie Larson as Carol Danvers, a pilot who gets incredible abilities.
It may not be obvious to anyone who isn't up on their Marvel news or superhero iconography, but this little sequence serves as a teaser for the upcoming Captain Marvel movie, which will mark the studio's first female-led superhero extravaganza - and potentially fill in some of Thanos' backstory while setting the stage for Avengers 4. As comics readers knew immediately after seeing the logo flash up on Fury's pager, that person is Captain Marvel. But in the film's post-credits scene, we see what we know has been coming for a while now: as Nick Fury disintegrates, he puts out a last-second SOS call to a mysterious person who'll definitely have a huge role in the Avengers sequel. Infinity War drops the mid-credits scene, pretty much because there's nothing left to say after Thanos does his Thanos thing and turns half of the universe's living creatures into frosted flakes.