You are averaging based on the population at the start of the experiment. In essence, you are counting dead people in your average, like Eliezer's offhanded comment implied he would

I consider every moment of living experience as of equal weight. You may call that "counting dead people" if you want, but that's only because when considering the entire timeline I consider every living moment -- given a single timeline, there's no living people vs dead people, there's just people living in different times. If you calculate the global population it doesn't matter what country you live in -- if you calculate the utility of a fixed timeline, it doesn't matter what time you live in.

But the main thing I'm not sure you get is that I believe preferences are valid also when concerning the future, not just when concerning the present.

If 2014 Carl wants the state of the world to be X in 2024, that's still a preference to be counted, even if Carl ends up dead in the meantime. That Carl severely does NOT want to be dead in 2024, means that there's a heavy disutility penalty for the 2014 function of his utility if he ends up nonetheless dead in 2024.

Of course, if we counted their preferences, that would be a conservatizing force that we could never get rid of

If e.g. someone wants to be buried at sea because he loves the sea, I consider it good that we bury them at sea.
But if someone wants to be buried at sea only because he believes such a ritual is necessary for his soul to be resurrected by God Poseidon, his preference is dependent on false beliefs -- it doesn't represent true terminal values; and that's the ones I'm concerned about.

If conservatism is e.g. motivated by either wrong epistemic beliefs, or by fear, rather than true different terminal values, it should likewise not modify our actions, if we're acting from an epistemically superior position (we know what they didn't).