You can use youtube-dl[1] to download videos from YouTube (and transcode them to free formats) without runnning their proprietary JavaScript. There's even a free software Android project[2] that lets you stream videos without running their proprietary JavaScript -- and in my opinion it's much better than the proprietary YouTube app.

RMS's concern with YouTube is that they use proprietary JavaScript and they use patented media formats. However, RMS has always made it clear that if you have the ability "at hand" to rip DVDs then purchasing a DVD is not unethical. Both of the pieces of software I mentioned allow you to avoid running proprietary software.

His other concern is that RMS wants people to publish things he appears in in a way that is much easier to access with only free software. The Internet Archive and CCC provide video hosting that is completely free software, with non-patented formats like VP9, and is easy to use. The idea is to make those forms of publishing more widely known about.

[1]: https://github.com/rg3/youtube-dl [2]: https://github.com/TeamNewPipe/NewPipe