Right, but the point is not for us to learn whether AI is an existential risk. The point is to find out whether mainstream academic AI people (and others) think it is. It's an attitudes survey, not a fact-finding mission.

How useful are these surveys of "experts", given how wrong they've been over the years? If you conducted a survey of experts in 1960 asking questions like this, you probably would've gotten a peak probability for human level AI around 1980 and all kinds of scary scenarios happening long before now.

You seem to presume that the quality expert opinion on a subject is somehow time/person invariant. It seems fairly intuitive that we should expect predictions of technological development to be more accurate the closer we come to achieving them (though I would like to see some data on that), as we come to grips with what the real difficulties are. So yes, the predictions are likely going to be inaccurate, but they should become less so as we better understand the complications.

A prediction of "it's going to happen in 20 years" from a researcher forty years ago when the field was in its infancy and we had very little idea of what we were doing is not as good as a prediction of "given all that we have learned about the difficulty of the problems over the last 20 years, it's going to happen sometime in the next few decades".