I know, but what about when I have several subtitle files? Different languages, or maybe several subtitle files I downloaded and want to check which one matches my video? mpv has zero flexibility.

With VLC I can just “Subtitle / Add subtitle track” or add the language code after the filename (video.en.srt, video.fr.srt, video.spa.srt), with mpv: just one file at a time: rename, launch, retry.

Until you want to add external subtitles