It's literally how FX companies and game development companies work - they hire staff for a project, then fire them once the project is finished, then hire them and/or new people if/when they get a new project. It's absolutely ridiculous but that's the society we live in.
I don't agree that Terraformers should not require population if they are inactive as now with the new orbital mechanics, it is possible that a planet needs semi-continuous terraforming work. Plus, I like to RP that the atmosphere on terraformed planets need constant maintenance, hence I keep a couple of the installations around for that purpose at each colony. Unless there should be a toggle for them, like with other facilities, where they are 'turned completely off'.