Of those downstreams who ever switched to libav, most have switched back to ffmpeg, incl Debian and now Handbrake. The development of libav slowed to essentially a halt with next to no activity in their git, and their latest release was 11 months ago, while ffmpeg had two major releases (4.0, 4.1) and numerous point releases in that period.
libav, in my humble opinion, also suffered from NIH where they sometimes - with what looked to me like religious zealotry - insisted on doing stuff own their own, while ffmpeg at the same time happily merged tons of changes from libav. This gave ffmpeg, in addition to their brand recognition and associated network effects, yet another advantage.
It also didn't help that libav created a lot of bad publicity for themselves, starting with them trying to take over ffmpeg in a hostile manner initially instead of forking, or when their debian packages (maintained by a libav member IIRC) started telling users that ffmpeg is "deprecated". Or when one of their folks sent ffmpeg a "cease and desist" letter claiming copyright of the logo they had been using for quite a while, insisting ffmpeg stop using it. (That's why libav still uses the 2d zigzag logo, while ffmpeg switched to a 3d zigzag one).
This describes the entirety of the libav initiative from birth to death. Good riddance.
I'm surprised it took handbrake so long.
I say that because mpv is one of the projects that mostly use parts of FFmpeg internals and also one of the first projects to use modern futures (and deprecate older versions too).
Do anyone know who won the fight of this fork?