I think that the idea of spanking replacing jail is very unlikely, but doesn't sound as absurd as most of them on the list.
The thing that makes it sound absurd is the idea that our grandchildren will find it EVIL to put criminals in jail instead of spanking them. Imagine people 100 years from now pointing to revered historical figures and saying they supported the use of jails, the way people point out that some of America's Founding Fathers owned slaves.
Although... if you accept the premise of other punishments replacing jail, I wouldn't be entirely surprised by the idea that people would come to regard jail as evil, and that many people would be reluctant to accept it even in a discussion of situations where the replacement punishments weren't practical. I think that also makes a rather useful way for me, personally at least, to consider the idea of whether modern ideas might not actually be better than past ones in a different light. In this thought experiment, our descendants genuinely do have a society better than ours, but their moral standards that have resulted from it aren't necessarily better than ours, even though they think they are.
Suppose I told you that I knew for a fact that the following statements were true:
You’d think I was crazy, right?
Now suppose it were the year 1901, and you had to choose between believing those statements I have just offered, and believing statements like the following:
Based on a comment of Robin Hanson’s: “I wonder if one could describe in enough detail a fictional story of an alternative reality, a reality that our ancestors could not distinguish from the truth, in order to make it very clear how surprising the truth turned out to be.”1
1Source: http://lesswrong.com/lw/j0/making_history_available/ewg.