i think the original proverb is actually the fish stinks from the head, which later became the fish rots from the head, but this is biologically incorrect.
But you can also see that maggots start in the ear, i don't know enough about Fish-gut bacteria, but you can check a Fish freshness when you smell the Gills the Colors of them and if the Eye is milky...well and sometimes the slime on the fish-skin....maybe that's the history of that proverb (freshness indicator mostly from the gills/eyes...head)
yea, visibly it looks like its starting from the head, and probably the smell comes from there as well, and of course thats where insects can start eating first (eyes, mouth etc)
it is funny though, i have used this proverb hundreds of times, until today i decided to think a bit about it, and look up how bodies decompose.
i wonder if in those big companies actually the leadership takes all the fault, maybe like in 1984, the real power is in the proles, they can change everything if they want.. they just dont seem to want change.
https://australian.museum/learn/science/stages-of-decomposit...
for example you can see the decomposition of a pig, it looks like the head is first, but it is actually the bacteria in the gut.