But piracy demand also indicates to companies an untapped market that they could tap if they create the right product

I agree with your conclusion of not watching it, but the reason that companies might observe and take a non-purchase as some kind of hypothetical market signal doesn't seem like the strongest argument for that, especially if people are avoiding the purchase specifically to deny them money for ethical reasons, that's not something they can resolve by making 'the right product'.

If pirating a Harry Potter show provides support to its author, I think that support is going to be more in the form of cultural relevance and mindshare than anything directly financial. Being a fan of an IP gives other people a reason to engage with it because your interest signals things to other people, like it means that by not watching it they might be left out of the loop etc, and since the law enforces property rights over what can be done with it and who can profit, that adds to the power of its owner by influencing the choices and interests of other people in their favor.