A lot of them have sponsors which pay more than the ads, or are on Patreon, or are also on other platforms that pay them a higher proportion or allow videos that risk demonetisation on Youtube, or sell merch, or something else.

Sure, and if you're a patreon supporter, or support them on a non-YouTube platform, great. But if they're monetized and you're just watching them on YouTube, which probably 90% of people do, then running an adblocker is preventing them from earning money they would otherwise have earned. Whether or not they're _also_ earning money via other means is irrelevant.