It’s not a 1% increase in fuel costs. It’s 1% of 3% (for 80% mitigation) to 17% (for total mitigation). That’s a 0.03% to 0.17% increase in fuel costs.
They'll all need to do it at once though, or people will just pick the cheaper flight that doesn't go around the contrail-forming region, basically every time.
Of course it's a coordination problem. It probably needs to be a regulation before it will actually happen.