The standalone films continue doing generally bigger and bigger box office by comparison. The Avengers films absolutely should be outperforming the others.
Eh, maybe? Black Panther, Captain Marvel, and Spider-Man are all outliers IMO -- cultural reasons for the first two, and Spider-Man has been a massive property on his own for decades. Other recent standalone films like Ant-Man and the Wasp and Doctor Strange have performed adequately, but not exceptionally -- they've been significantly outperformed by movies like Aquaman. Even Thor: Ragnarok, which is one of the best films in the entire MCU, didn't make as much money as the apparently terrible Venom movie (although, to be fair, that property is associated with the Spider-Man juggernaut).
It's hard to draw any major conclusions, though. Iron Man and Iron Man 2 are two of the lowest grossing films in the MCU, and then Iron Man 3, which was better than 2 but not as good as 1, brought in an absolutely massive haul that dwarfs most of the other standalone films; it made more money than every MCU film except Black Panther and the four Avengers films.
I still expect movies like Shang-Chi and (especially) the Eternals to bring in results closer to something like Doctor Strange than something like Black Panther or Spider-Man, although Shang-Chi could get a similar cultural boost.
Also, of course Endgame made a tremendous amount of money. It was the culmination of a massive 24 film arc. There's absolutely no chance any new MCU film in the next few years comes close to the Endgame box office total.
This is a strange conversation for me to have, because I don't think the MCU is going to suddenly fall off the face of the earth or anything. I think it will continue to be a successful money maker for Disney. But some people are looking at the Avengers films and considering those a barometer for all future success and that's just not the case. At least until they bring in the X-Men, which could probably rival the Avengers if done well.
