The thing with Cassie is that the 5 year jump affected everyone who wasn't snapped. It wasn't done with the only purpose of age her up.
If they do something like this again, just to have a male Black Panther, that would be completely stupid IMO.
Now, I'm not saying this is the case with everyone. Some people have genuine reasons to not be convinced of Shuri as BP.
But I'm also seeing other kind of people who, it seems, just don't want a female Black Panther. We literally just saw Shuri's "origin story". Her character story has now really begun. And people are rushing to say "Well maybe M'Baku can now be Black Panther", "Well, there's the 6 year old T'Challa. Let's find a way to age him up for the next film so he's an adult".
We have a Black Panther. No need to find another.
She was very much the main character in this movie, beginning to end, and she did quite fine.
Can't wait for Black Panther 3 to continue to see her story, and her bond with King M'Baku.