HBO Harry Potter is going to set records on an astronomical level, and I imagine more than half the people reblogging you are performative cowards and will watch it anyway directly on HBO. (I say that as anti HP and JKR). Pretending like it’s not going to be the most successful show in HBO’s history is insane and really underestimating it. Boycotts are not going to work because it is a tiny blip of people willing to do so. What do we when it is mega popular and people continue to love and enjoy her work?
So I wouldn't even answer this ask EXCEPT it's a fantastic example of someone who hates you and wants you to undermine your cause pretending to be on your side, so let's go through the points.
1.) Makes the assertion that you've already failed eight months in advance. Wants you to give up and give in because you think the cause is already lost.
2.) Implies that everyone else is secretly gonna do it, so you may as well too. Wants to chip away at your resolve.
3.) Claims to be on your side and therefore a trustworthy source.
4.) "Boycotts don't work." Demonstrably they do, as long as people are organized and persistent. Look at how Target and Starbucks are sweating and begging people to come back. Boycotts work.
If someone comes to you doing this shit, they are not your ally, they're trying to mess with you. They want you to fail. On the bright side, they're also often an indicator that your cause has gotten big enough that they're worried enough to go about it all underhandedly, so yay?