I worked in a shift system that necessitated a lot of overtime to make it work. There were always those that said yes, and those that said no. The ones that said yes made a lot of money, which is why they said yes (apart from some professional pride in maintaining the service they were trying to provide). Eventually, enough of even those people said no. The company then couldn't provide the service, suffered operationally (thus financially and reputationally), and they cracked and employed more staff.
If ATC are expected to work 3 of their 4 days off and you want that to change, say no to the overtime. Has to be everyone though, because as long as enough keep saying yes (which appears to be the current situation), things won't change.