Quite frankly a substantial amount was needed to effect the sea change needed in passenger treatment by airlines in America. I would imagine Munoz (he lost the upcoming Chairmanship position already) would want to take charge and put this thing to bed ASAP.
The amount has to have been large for Dao's lawyers to stop all other action against everyone else they could have charged.
Munoz did the right thing here, to have United take the fall on behalf of everyone else.
United was in a "no win" situation that could have only gotten worse as the wound got deeper and deeper and more infected.