Giving other people a to default disable a thing does not force you to do it, too. Giving different people options about how they want to experience the Web would be the entire point.

The media.autoplay flags in about:config provide that option for anyone who wants it (and even more granular controls than those being exposed in the UI here).

Some big browsers need to disable auto-play by default to force websites to actually support commencing and pausing video in terrible tiny javascript video players.

They don't work. I have all of those flags enabled and sites like CNN still autoplay videos. With sound, too, as a matter of fact.

If it's already not possible to implement perfectly, what is being argued for in this thread exactly?

How would it negatively impact you to click / tap on a video to play it?