So nothing is going to happen to them.
You're right to be pessimistic, but wrong to be fatalistic. A citizen can:
Contact their Members of Congress and demand:
--Defund any census that asks about citizenship or pass a provision in the Commerce's Authorization Act prohibiting it from asking about citizenship;
--Hold oversight hearings on Department of Commerce's/DOJ's procedures for establishing Census questions pre/post SCOTUS decision;
--Refuse to confirm any political appointees that do not swear to follow SCOTUS decisions;
--Direct USPS to not mail citizenship questionnaires that contain the citizenship question.
I could go on, and of course, Congress could impeach any official (and citizens can contact their Members of Congress to demand such action). If millions of Americans did, elected officials would perhaps not shirk the will of the People so easily.
Separation of powers is a beautiful, and imperfect, thing. It's intended no separate yet co-equal branch of government can "govern" [legislate/appropriate, execute, and interpret] on its own. (Of course there are smaller versions of this in federal agencies, but an executive department cannot go against the expressed statutes of Congress or expressed decision of the federal judiciary). But it requires basic civics on the part of the electorate and the elected, as well as faithful service of those elected and unelected officials in government.