Technically nothing.
Morally, you are depriving the content creators and Youtube of revenue (if they rely on Youtube ads). You could argue that they aren't somehow fundamentally entitled to make money, but you are also not fundamentally entitled to consume their content. The content creator had to do work to produce the content and YT has to do work to provide a platform where it can be disseminated. Naturally, they want to be paid for doing this work. We can argue about how much they should be paid, but again nobody is forced to watch YT. It's a voluntary transaction. If you don't like ads, buy a YT subscription. If you don't want to support YT, encourage your content creators to provide their content elsewhere.
Practically, some companies have been historically willing to shut one or both eyes over open source/free projects as a gesture of goodwill (or marketing exercise). The moment money is involved, the lawyers tend to be unleashed and the whole "this is just a free hobby project with no monetary incentives for the developers" goes out the window.