Is there a reason why Mozilla haven't used their position and funding to just hoover up all the most successful open source projects to create a more cohesive ecosystem? Surely any addons - like uBlock Origin - which have massive usage should just be baked in to the browser by this point? The likes of Nextcloud and Mattermost would also be great additions in trying to create a true rival to Microsoft and Google's off…
Why would Youtube, which makes money off ads, allow a web browser that blocks ads to access Youtube when they could just refuse to serve their user agent?
It's difficult for me to envision how that's rigorously enforceable to prevent something outside of the app (like a browser) to access something ultimately consumed by an API without resorting to (expensive and crackable) digital rights management at the hardware level. You can run down whatever train of thought you want, but ultimately since the client is running on machines controlled by the consumer, it can be cracked, reverse engineered, decapped, or whatever. This has been the end state for every attempt the industry has made to control user behavior. DVD CSS, Bluray, Sony's CD rootkit nonsense, every game console, satellite TV cards, analog cable TV, on and on, all broken.
I'm confident google/youtube wishes there weren't applications like FreeTube and yt-dlp. The fact of the matter is it's practically impossible to exclude them but still keep their own ecosystem functioning at an acceptable cost and with consumer buy in.