The problem is, booing can work. See the Moura sub and Nuno. It precipitated a change (at least symbolically, you assume the wheels were already in motion).
I don’t boo, and I don’t like the fact Sanchez was the target and he’s way, way down the list of targets.
But, for example, I am surprised that Son never got some stick. For all his heroics, he’s been absolute toilet for most of the season. There have been collective groans when his name is on the sheet again. Booing him isn’t booing the player, it’s telling the management and club to get their shit together and put him on the bench.
A lot of people laugh at the threadbare ENIC demonstrations. Now, in contrast, it seems the fans are being told off for the only legitimate way they can vent their frustrations.