If it's common knowledge that every eligible voter is using UDT I think the outcome might be that everyone chooses a mixed strategy: vote with probability p (for some fairly small p like < 0.1) and stay home with probability 1-p. This way, the outcome of the election is almost certainly the same as if everyone votes, but its cost is much smaller.

Caveats: I don't know how to derive this mathematically from the stated assumption, and I have little idea how to apply this type of reasoning to humans. Actually it still seems plausible to me that E(total number of votes | I vote) - E(total number of votes | I don't vote) is near 1 and therefore CDT-type ("deciding vote") reasoning is a good approximation for my actual situation.