To return to the ethnicity of Hermione:
Something strikes me as slightly patronising in saying that "well, Hermione can be non-white if that's how you want to imagine her". It's a little bit "yes, dear". I find JKL's reaction a bit off for similar reasons. I don't expect that she gave Hermione's ethnicity any thought and therefore - because of her own background - imagined her as white by default. Her response when the issue was raised suggested as if she thought she'd better get with the programme, and so it seems disingenuous to me.
For myself, the reason why I roll my eyes a little at the issue is because it's as if some people want one of the main protagonists of the book to be non-white in order to make things fair. In my view, they have alighted on Hermione because it clearly isn't possible to do the same with Harry or Ron because they are more completely described. I wonder if, in time, someone will point out that it also isn't fair that of the three main protagonists two are male and only one female, and therefore one of them - presumably Harry - must identify as non-binary in order to even things up, as there's nothing in the book to prevent this conclusion.
What matters is what JKR wrote in the book, and those who say Hermione can be imagined as not white are asking everyone else to ignore her description in the book as pointed out above.
There are probably a good many people - and to be honest I am one - who read books like Harry Potter in order to escape the real world, and who therefore don't apprciate the real world intervening in the form of identify politics.
There's a much better non-white protagonist in A Wizard of Earthsea anyway.