Plumbing could really use a buff since electrical offers so many extra benefits, there's almost no reason to get a plumber unless you already have an electrician.
To make plumbing more attractive, the upgraded latrine should offer not just a morale bonus, but also infection resistance. After all, what's more important to hygiene than a proper latrine?
Providing an extra infection resistance would also make the material cost of an upgraded latrine more worthwhile. It wouldn't even have to be a significant amount of resistance, either. Even 20 extra resistance would make plumbing more attractive, especially to a community without a medical skill in a lethal zone.