Everyone forgets about the live action Inhumans...That's not true. Incredible Hulk, Iron Man 2 and 3, Thor 2, GotG2, all of them were considered questionable-to-bad films. They all made a profit and have fans, I don't think Marvel has had a straight up flop yet (maybe Eternals?), but there were always movies that weren't great, that had people saying "the MCU is dead now!!!"
There have *always* been clunkers in the mix. Of the first three MCU movies, two of them are considered among the worst. And that's not to discount the unparalleled success the MCU has seen, what it's done is unprecedented and impressive beyond words. But let's not pretend that they only started making shaky movies after Endgame. The only difference is, we're not giving them a free pass anymore.
And I find the claim that Carol's film would have been just as successful if it had had Mar-Vell to be rather....not at all accurate. You gotta ignore a whole lot of stuff to make that claim. It's true that Carol struggled in the LCS when she became Captain, but everywhere else (except fox news I suppose) the move was celebrated and became a huge deal. Did Marv stay on the top of the trade charts month after month? Did he have legions of fans invading conventions dressed as him? Was he getting larger media attention and pushed as a poster child for a social movement? No. Carol did. She got all that and more when she became Captain.
I don't think any of us liked the forced, artificial push that Carol got. Marvel didn't try to make her A-list because of fan demand, but because they decided she was their best shot at having a high-profile female lead. And a lot of folks resented that, understandably. But despite some people not liking it, Carol becoming Captain was a huge moment that captured lightning in a bottle and became indicative of something bigger in the socio-cultural mix. This is what kicked off the diversity push of the 2010's, which led to, among other things, Kamala. Can't really denounce the importance all this carried or the impact it had, and the fact that Carol was the first female-led MCU film certainly didn't hurt her box office either.