I mean a normative theory (or proposal if you prefer). Utilitarianism clearly fails as a descriptive theory (and I don't think it's proponents would generally disagree on that).
A normative theory that proposes everything would be fine if we could all just agree on the optimal outcome isn't going to be much help in resolving the actual ethical problems facing humanity. It may be true that if we all were perfect altruists the system would be self consistent but we aren't, I don't see any realistic way of getting there from here, and I wouldn't want to anyway (since it would conflict with my actual values).
A useful normative ethics has to work in a world where agents have differing (and sometimes conflicting) ideas of what is an optimal outcome. It has to help us cooperate to our mutual advantage despite imperfectly aligned goals rather than try and fix the problem by forcing the goals into alignment.
Utilitarianism is a theory for what you should do. It presupposes nothing about what anyone else's ethical driver is. If cooperating with someone with different ethical goals furthers total utility from your perspective, utilitarianism commends it.
My girlfriend/SO's grandfather died last night, running on a treadmill when his heart gave out.
He wasn't signed up for cryonics, of course. She tried to convince him, and I tried myself a little the one time I met her grandparents.
"This didn't have to happen. Fucking religion."
That's what my girlfriend said.
I asked her if I could share that with you, and she said yes.
Just so that we're clear that all the wonderful emotional benefits of self-delusion come with a price, and the price isn't just to you.