Cynicism means a distrust of the goodness of people.
Changing the SOP based on customer feedback does not make the your service stronger. A desire to change is yielding to cynicism.
To now, because one person complained that the he didn't realized there was egg whites in a sour. That everyone onwards must be informed.
Allowing pass experiences to ruin future decisions is a form of weakness. Weakness in our leadership, because we made the service worse for everyone else all because one person's complaint made us insecure. Weakness is not cool.
It breeds insecurity in your staff, because now you implement a rule to inform everyone that there is egg white. This implies to your staff that they might get in trouble if someone finds something they weren't expecting in their drink.
It also belittles the good guests who obviously know sours often come with egg white (traditionally it doesn't, but we are not in the 1800s).
Unhappy guest do not actually tell us much about our service. All it tells us was that they didn't like us that one visit. Work long enough in the industry, and you will see guest who come back even though they complained before.
If they don't take their anger so seriously, why do we do it for them?