@MasterBeth Why are you assuming the canonical POC characters are not beloved, for a start. If you were a long term, passionate HP fan you would know otherwise. Secondly, it's about fidelity to the canon across the board, and you are the one deciding that the key focus is wHiTeNeSs, when it's not at all. There were infinite screw ups with casting the original films when it comes to this exact issue too, despite all three of the trio being played by white actors: none of them looked faithful to the source. Exactly capturing my point. A casting can be white and yet totally wrong.
There are other subtle issues that come into play when they "race bend" a character, and the problem with these is not at all related to whiteness but to the harm that textual accuracy would cause to a child actor. For example, I think that out of the trio, Hermione would be by far the best character to cast with a black actress - her appearance is less of a focus in the books (colour wise) and "black Hermione" is historically loved by the fandom. It may not be accurate (it's not) but there is strong and much loved precedent in the fandom and in the quasi canon of Cursed Child. However, then you run into the issues of (1) her hair, and how to deal with it accurately when it's supposed to be very frizzy, messy, wild etc. That could obviously be hurtful for a black child actor. And also the fact that Hermione is canonically one of the less "attractive" girls in the year, has big teeth for the first years at Hogwarts, etc. They would essentially need to cast a "less pretty" child who would potentially even need small prostheses. Minor "uglification" of a black actor is clearly more fraught and controversial than a white actor, especially when a young actress playing black Hermione would likely receive harassment anyway, from people who saw the casting as "woke" and weren't aware of the full HP background (the concept of "black Hermione" originated with fans, before Cursed Child).
I could go on, believe me. I have a lot more to say on this. But it's so tedious talking to people like you who just lazily assume the worst all the time and never consider there might be more nuance in a situation.