Black Panther was a superhero created pre Black Panther party by a bunch of white dudes. Also it takes place in Africa, so I think it would be really weird if they are all white.
Sure, but change the setting to US, Canada or Sweden and you'd have an outcry. It doesn't matter, the idea that a movie that advertises how it's deliberately all white-cast would get killed.
I've always hated this argument. It sounds reasonable on the surface, but breaks down the second you put even a moment's thought into it.
Yes, it would be terrible to make 'white panther', just like it would be terrible to make a Straight Pride Parade. Why? because we live in a world where black folks, gay folks, women, and all sorts of other groups have been repressed, and these celebrations aren't about bringing down the majority, but lifting the minority.
Black Panther's cultural relevance wasn't hinging on the idea that white folks are inferior, but that Black people aren't. The Gay pride parade isn't about devaluing the importance of straight relationships, it's about empowering those in gay relationships.
It's the same way with comedy. You don't punch down, you punch up. It's okay to tease and belittle those in power because they can deal with it. Poking fun at the upper class is fine because they're the upper class. It's not okay for a rich white guy to make racist jokes about Mexicans or Black folks because it stops being about the humor and starts being about authority and oppression. While there are exceptions, of course, that is the reality of it.
That's why Black Lives matter is a significant cultural movement but White Lives Matter is just racism. It's about perspective, and if you see them as equals, then you're not living in the real world.