I’m not debating wether it’s moral or ethical, but if you were in their shoes trying to juice revenue to increase the value of your equity, you’d either charge a monthly fee for users to remove ads or fight like hell against people who are trying to download videos without ads.
That’s exactly what YouTube is doing.
FWIW, I can’t stand ads and I think it’s complete bullshit that they’re suing to take down youtube-dl, but they’re acting complete rational for the incentives they’re optimizing for. The problem is that optimization is at odds with users like ourselves.