(Seems I missed this conversation a year ago. Having worked with online streaming and TV tech and being a movie buff, I do have some opinions on this topic ...)
With modern TV panels in people's homes going larger but without homes or viewing distance increasing, we're transcending from the problem of letterboxed movies being too small on the screen towards the problem of "content" (movies/TV shows) not having been made to be viewed with such a large field-of-view ...
I think that people should always have a choice between watching in letterbox, or in "full screen"/"pan-and-scan" or anything in-between. Sometimes a middle-ground is the best, such as when 4:3 content gets some top and bottom chopped off, and there are narrow black bars on the left and right. (BBC has broadcast some older shows like this, and not many people noticed)
I think that movies distributed digitally should have dynamic pan-and-scan information inside the data stream that gets utilised by the movie player (but it would have to be done smart, and the result depends on the person doing the work)
BTW. My mind was once blown when I realised that the active area in 2.35:1 letterboxed in 1080p is only 460 pixels high. You could pan-and-scan this in SD 16:9 (480p) without losing vertical resolution and you would still get small black bars at the top and bottom.