Not if people have a strong preference to go on living.

The preference to go on living results in large part from the good things (i.e. opportunities for preference satisfaction) available in life. If we didn't care about those any more, the strength of the preference to go on living would presumably diminish considerably.

But yes, the policy recommendations of utilitarianism always depend on how the numbers actually come out. The point is that they're too dependent on a single parameter, or a small subset of parameters, contrary to complexity of value.

(I would go so far as to argue that this is by design: utilitarianism historically comes from an intellectual context in which people thought moral theories ought to be simple.)