People are blinded by a decade of success. Prior to 2008 the Marvel Trinity was Spider-Man, Wolverine (and by extension, X-Men), and Hulk.
Even the Avengers were second-tier to the X-Men. That's why Capcom didn't make Avengers vs. Street Fighter. Their Marvel fighting games started with the X-Men and then eventually added the second-tier characters.
I am not blinded by their success. I've been following the comics for decades now. Before the movies, the Fantastic Four and the Avengers were anchors in the marvel line up since the 60s. They had cartoons and lunchboxes before X-men ever became a thing.
Millions of people know who they (the Avengers) are. They are not obscure. I'm not going on the success of the movies, I'm going on the success of the comic. Only teenagers and comic noobs shrugged their shoulders when Iron Man was announced. Lots of people knew who Iron Man was and saw that pitch-perfect cast and wanted in. It did impressive numbers that opening weekend.
Again, they would have lead with Cap or Hulk had they re-secured the rights to their films first. But, since they didn't they rolled the dice with Iron Man. It was a gamble, but you're acting like Squirrel Girl got announced, everyone booed them off a cliff and then it made a billion dollars. That's not really how it went down.
You're right about Guardians, though. That was a much bigger gamble and it paid off. But, I'll add that that movie came out AFTER Avengers and the big 3 Phase plan leading up to Infinity War. Every movie, regardless of quality, has benefited from the phase 1 Avengers.
Now that Endgame is concluded, a lot of people left satisfied for now. Endgame had a send-off that rivaled Return of the King. Now Marvel has to start all over. And they're starting, NOT with X-men or Fantastic Four (or even Spider-man since he's gone back to Sony) but with Shang-Chi.
Don't get me wrong, I will see Shang-Chi but I've been collecting comics for a long time and I barely remember him. He is truly an obscure character. So are the Eternals- a team I am not familiar with at all. And maybe Marvel doesn't need it's comic collector core for these movies to work, I don't know. We'll see. I just know that lack of buzz, lack of word of mouth, burnout (or however you want to say it- lots of people think Endgame was a nice tidy bow to the Marvel Studios characters), AND a mediocre movie that doesn't deliver the goods will lead to a downturn in superhero movies.
If the movies are GOOD then people will keep seeing them, but if they falter at all. If the quality dips even a little bit, you're going to see the numbers drop. It might still make money, but it's not going to make Black Panther levels of money.
By the way, Dr. Strange and Ant-man are also relatively obscure and they didn't move the needle all that much, so Marvel isn't bulletproof with obscure characters.