With postgresql synchronous replication, in order to lose writes that have been acknowledged to the postgresql client, you'd have to lose filesystem data on both the primary and the synchronous standby. (I believe the way postgresql uses the term "committed", you can lose data that's "committed", but not once postgresql has acknowledged it to the client.)

For many applications, consistency includes not losing acknowledged data. If I PUT data into an application and fetch it back and it's not there, that's not consistent.