Earlier quoted context omitted.
You're mentioning the end user's learned expectation to get videos for free. But what about the creator's expectation to get paid for their hard-earned views, to then pay for the gear purchased and the production of the entire video, including team members? If you're really anti ads, don't take the content for free, find another piece of content that answers your ad-free philosophy.
I'm saying platforms can't have it both ways. If they want to avoid ad-blockers and video downloaders then pick another medium, or invent one and create a new thing that achieves this objective. The web is not TV. Public content on the web is public and user-rendered. That's what made the web and YouTube what it is today. YouTube has every right to create a proprietary YouTube client (which they do on mobile devices)…
Definitely not illegal to do so, so do as you please, but don't turn yourself into a white knight by some mental gymnastics with the sementics behind what a user agent is. You want to enjoy the video without ads, regardless of the consequences on the creator who made that video — and accessorily, the service hosting it and streaming it to you!