Saying "but you can't bring hurt down to zero" is an invalid objection because it is irrelevant, and a pernicious one, because people use that form of objection routinely to defend their special interests at the cost of social welfare.

You speak of "social welfare" as if it were an objectively measurable property of the real world. In reality, there is no such thing as an objective social welfare function, and ideologically convenient definitions of it are dime a dozen. (And even if such a definition could be agreed upon, there is still almost unlimited leeway to argue over how it could be best maximized, since we lack central planners with godlike powers.)

If we're going to discuss reworking of the social contract, I prefer straight talk about who gets to have power and status, rather than attempts to obscure this question by talking in terms of some supposedly objective, but in fact entirely ghostlike, aggregate utilities at the level of the whole society.

Also, referring to the "usual problems with utilitarianism and social engineering" literally says that there are problems with utilitarianism and social engineering (which is true), but falsely implies that (a) utilitarianism has more or even as many problems as any other approach, and (b) that attempting to optimize for something is more like "social engineering" than other alternatives.

I'd probably word it a bit differently myself, but I think (a) and (b) are in fact true.