You're definitely right that if we presume death is the shutdown of ALL systems then that answers the question. But the statements of Epicurus do not appear to be so sweeping - they focus on the end of "sensation" as sufficient to answer the question.
I think that the most obvious concern arises from the possibility that if Epicurus is focused only on "sensation" then what does that mean. Is the end of sensation sufficient to establish that absolutely no unpleasantness can occur after death? If consciousness exists totally independently from sensation, and an continue independently without any "new sensation" input, then the possibility would exist that consciousness could continue to rehash old experiences (old pains) from old sensations. If so, then someone might argue that death is not necessarily the complete insulation from all future unpleasantness.
There are various ways to deal with this question, but I do think the question is a valid one to ask. At least it will arise in the minds of some people given the way some people look at consciousness today as inexplicable and mysterious.