Web standards are open but that doesn’t mean you can’t build on top of them. YouTube is not an open standard. If Google wanted to end web support for YouTube and force everyone to use some sort of YouTube app even on desktop, they could do that. They’re not obligated to make sure YouTube works on EVERY browser especially when an insignificant amount of their users come from there.

A game running on windows isn’t a false equivalence either. A lot of games are built on open standards and open source (e.g. OpenGL, various game engines, etc) and still won’t work on wine/linux. If a bunch of people are using windows in order to play their games, that’s not too different from a lot of people using chromium browsers to use YouTube.

Also, I work at Mozilla.