It seems that you are expecting a situation somewhat like this:

Day 1: I expect to be done in 5 days. Day 2: I expect to be done in 5 days. Day 10: I expect to be done in 7 days. Day 20: I expect to be done in 4 days. Day 30: I expect to be done in 5 days.

Basically, this cannot happen if I am updating rationally. You say, "Worse, each additional step is novel; the additional five steps you discovered after completing step 6 didn't add anything to predict the additional twelve steps you added after completing step 19." But in fact, it does add something: namely that this task that I am trying to accomplish is very long and unpredictable, and the more such steps are added, the longer and more unpredictable I should assume it to be, even in the remaining portion of the task. So by day 30, I should be expecting about another month, not another 5 days. And if I do this, at some point it will become clear that it is not worth finishing the task, at least assuming that it is not simply the process itself that is worth doing.