But that's my point -- Iron Man, Thor, and Captain America were not A-list characters in the comics. Spidey, the FF, and the X-Men were the A-list, and maybe the Hulk. But the MCU didn't have the rights to them, so they had to start with the B- and C-list characters that mainstream audiences had barely heard of, and they made them A-list characters through the quality of the movies, which completely disproves your argument.
I mean, seriously, where did this ludicrous idea come from that it's impossible for a movie to succeed unless it's based on already famous characters? Nobody had ever heard of Luke Skywalker before 1977. Nobody had ever heard of Indiana Jones before 1981. Nobody had heard of John Wick before 2014. Good movies make their characters famous. That should be obvious, since every character had to start somewhere.