So not only would ffmpeg have multiple uncovered vulnerabilities, they would have less contributions and patches and less money for funding the maintainers. And for what? To satisfy the unfocused and mistaken rage of the peanut gallery online?
That's only one possible benefit.
Another could be to gain leverage on big tech companies via dual licensing. If Google, Amazon, etc want to continue using FFmpeg without forking, they could do so by paying for the old LGPL license. It would likely be cheaper than maintaining a fork. They'd also have to release their changes anyways due to LGPL if they ever ship it client side.
So the incentive to contribute rather than fork would remain, and the only difference is that they have to pay a licensing fee.
Ofc this is probably just a fantasy. Relicensing FFmpeg like this probably isn't easy or possible.