Hi yaddoe, the verse does not say it is for the "deads sake," it is for or on account of coming into contact with the dead. The washing or baptism was for the person who became defiled regarding or concerning coming into contact with the dead. The Greek word "huper" does not necessarily mean in one's place or stead, it can mean "regarding," or "concerning."
G5228 ὑπέρ (huper)
- a primary preposition;
- "over", i.e. (with the genitive case) of place, above, beyond, across, or causal, for the sake of, instead, regarding;- with the accusative case superior to, more than;
KJV: (+ exceeding, abundantly) above, in (on) behalf of, beyond, by, + very chiefest, concerning, exceeding (above, -ly), for, + very highly, more (than), of, over, on the part of, for sake of, in stead, than, to(-ward), very. In the comparative, it retains many of the above applications.
When Paul spoke of what shall "they" do, he was referencing the Jewish people who go through this ritual of washing or baptizing for or regarding contact with the dead. His reasoning is WHY would they even have to do something like this (cleansing from contact with the dead), if there is no Spiritual or ultimate cleansing from death, the resurrection. Hope this helps. KB