It's not piracy. You might have a problem with it ethically. But you're not breaking copyright laws by blocking ads.

Another way to look at it is additive rather than subtractive. If I visit a site with a text only browser that cannot display ads, what is your position then? And if I then implement the ability for my browser to play only the main video on any page, what then?

When it comes down to it, we have no obligation to view the content on a webpage the way the publisher of said webpage wants us to. You can think of plenty of other examples that make "adblocking is piracy" ridiculous - I invert the colors but the publisher doesn't want me to see it with inverted colors. I wear sunglasses while looking at it, which changes the way it looks. Maybe the site I use always puts an ad in the same place so I stick a bit of tape on my monitor in that location, is that bad?