If I recall correctly, for a brief time mplayer disappeared from Debian. At the time I evaluated various alternatives and mpv was one of them, but I switched back to mplayer as soon as it made its way back to the distribution. For me, mplayer is the standard media player. I guess I just don't see any reason to switch to mpv or to any other player.
I see no reason to cling to mplayer/mplayer2, mpv has pushed things forward from there.
mplayer is what I used to use ages ago, can't think of a single thing I miss that it did which mpv doesn't. Like you, I was introduced to mpv through the debian disappearance, and it's been fine. Forks can be necessary to keep development active; it's one of the core features of FOSS.
