Pshaw! I think you need to pay closer attention to modern cinematography, and notice how often an expert cinematographer will frame the important bit of the shot in a doorway, between a wall and a truck, through a keyhole, etc. Not to mention the sadly lost art of split-screening, where the director will often just ignore the artifice of setting up the scenary and props to frame the subject and just accept the “black space” — which, let’s face it, works a heck of a lot better in the cinema than on a web page.
It would be nice if YouTube used something other than black to frame their portrait format videos, true, and I don’t know how you can regard cropping the video as any kind of solution, but what I’m proposing is simply to allow non-standard frames. YouTube is not a television, there’s no physical requirement to fit all video into a fixed format. Heck, even cinemas have been able to adjust their landscape screens to more than one landscape ratio for decades, and they have to physically move atoms around. All YouTube have to do is change a bit of code. An involved bit of code, to be sure, but in the end no more difficult to accommodate than non-standard frames for static pictures.
These folk who turn their camera or phone sideways to take a shot that suits a portrait format are applying instinctively the same visual reasoning to moving images that any photographer would. If your object of interest is a person instead of a landscape, then you use portrait format. Simples.