YouTube is already actively blocking alternative clients. that's why yt-dlp needs a JavaScript runtime these days: https://github.com/yt-dlp/yt-dlp/wiki/EJS

They are also not averse to using legal means to block them. For example, back when Microsoft shipped Windows Phone, Google refused to make an official YouTube client for it, so Microsoft hacked together its own. Google forced them to remove it from the store: https://www.windowscentral.com/google-microsoft-remove-youtu...