I don't think Wizards are actually going to do a Harry Potter set for Magic The Gathering, despite rumours (Hasbro has the license for the toy line for the franchise, for example), but if they did I would simply never ever purchase a single product from them ever again.

It's not that hard to proxy everything. If anything it's easier than getting the cards normal style. I don't do it now, because I like the cards and I like having real things I can trade if/when I'm done with them, and I also really enjoy attending Draft and Sealed events. But they'll all be of zero value if the parent company decides to partner with someone who wants to kill me.

They will lose thousands of players and, if my LGS is anything to go by, attendance at their events will halve. Trans people are a substantial portion of the current player base for mtg - I don't think they would court such an obviously ultra-controversial partnership when this is already their golden goose. Hasbro needs the playerbase to not all leave to go play one of the dozens of other less popular CCGs, or to just stop buying products and proxy everything instead.