I am not really going to write a long history of marvel, their comics, their bankruptcy and the Disney purchase. we know their story well and I usually don't tend to have reservation about ''popularity'' as not only can it be shallow but popularity never tends to be permanently fixed because the world is ever changing and artists, products or franchises tend to adapt to new styles to maintain relevance or longevity and this is where I draw the line with some Marvel series.
You can make the case Marvel characters are ''popular'' now like Black Panther, Antman, Avengers, GOTG, Shang Chi because of MCU movies and the Disney empire as well as the Media that supports them immensely.However one thing some Marvel characters has not shown is longevity. Longevity is what Spiderman, X-MEN and I will say in a small doses Fantastic 4, Hulk and even Blade has had that other characters don't have yet and if MCU were to be gone tomorrow or start to fade away in box office popularity, since you can make a strong almost perfect case the MCU films does not really have any artistic popularity, given many film makers have dismissed them the recent of those is Denis Villeneuve.
How much will characters like GOTG, Avengers, Ant Man, Black Panther, Thor still be relevant? Where will these characters be in 20-40 years without the MCU? Spiderman X-MEN F4 Blade have all managed to stay relevant for almost 60 years without needing the MCU for help. Disney could not even erase them completely when they tried in the mid 2010s because other film studios still owned the rights and that is a true sign of their longevity and I would even argue that right now the MCU does them more harm than good because the MCU movies fall under the formula of Monopoly film style that is actually immensely unpopular in the artistic circles of film making.
All of these been said, I will put my money on Black Panther that will have longevity once the MCU is over completely, 10 years ago I may have said Iron Man but I think Disney dropped the ball with this IP by not choosing to see that Iron Man and Batman had some similarities and Iron Man needed to be expanded beyond the MCU narrative.