The thing is each aspect ratio has its merits for productivity, so 16:9 can’t be an end-all be-all.
Also, what I’m talking about is different from what you’re talking about. If source footage was shot in 2 aspect ratios, and both are used in a YouTube video or movie, it should be possible to label video segments with their correct aspect ratio. Right now, a single video can only have 1 aspect ratio, so if a video was formatted for 21:9 with some source footage being 16:9, if you were to watch that video on a 16:9 monitor, you’d see black bars on the top, bottom, left, and right of your screen despite the video segment being 16:9. If you watch WandaVision on a 16:9 or 4:3 monitor, you’ll understand what I mean.