I would be careful comparing casual movie goers to comic book fans. Comic book fans are generally far more invested. I’m not saying it’s a foregone conclusion that these recasts are going to fail. But it’s open question, where as in the past it was a foregone conclusion.
The movies breaking billions at the box office are largely composed of casuals like myself. I read manga, but never read a single comic book in my life. It's not my thing. But the MCU is very much my thing. The best movie-going experience for the buck, at the moment. I don't expect recasts to impact viewership much at all, so long as the recasts make sense. Swapping out Brie Larson, without the role of Captain Marvel being handed off, would be a poor casting decision. Having someone else take the mantle of Ironman, or Captain America (which has already happened), should be totally fine. One died, and the other passed the shield.
To put it in perspective, the toxic comic book fandom complained bitterly about Brie Larson, and Captain Marvel. However, the casual fandom powered that film to box office heights. With Coogler handling BP2, I think we can expect that the transition can be handled well, to see that make some good money too.
What will impact box office appeal more than anything casting or story related is COVID. That's going to continue to impact box office revenue globally, and well into 2022 at the rate we're going. So I don't expect any billion dollar bangers in Phase 4, and it doesn't have to do with the lack of appeal for the new stories. I just don't think people are in a big hurry to crowd into stale air to watch a movie anymore. Streaming movies has been perfectly fine for the past year. I see no reason personally to ever watch a movie in the theater again. Not until we're free and clear, which we are most certainly not right now. I won't go back until I can enjoy my popcorn and big gulp mask-free.