As for Farming, yes - we were once a nation where many of us, most of us, were farmers.  Family farms.  And farming has since become more "efficient" with multiple farmers being replaced over time by fewer and fewer farmers on larger and larger farms.  But that process has also been inherently problematic.  It turns out that this sort of 20th century "efficient farming" with its fertilizers and pesticides has actually been ruining the soil and poisoning the aquifers and toxic to biodiversity.  It turns out that we need to pay more attention to the health of the soil (as a living thing) rather than treating it as a commodity, that agroforestry is infinitely better than uninterrupted acres of monoculture plantings, that fertilizer and pesticides both are harmful not only to the soil and to aquifers but also to the environment as a whole, and so on and so on.  Also, with regard to smart automation in general (i.e., the 4th industrial revolution), eventually the robots will not only be able to do anything and everything that a human can do, but also do it better.  Hence, for example, given these developments, it's very possible that we might once again become a nation where many or most of us are farmers on small family farms - whether as vocations or avocations.  In short, the agriculture example utilized by Krugman, while interesting, does not take into account all the pertinent variables.  I'm in basic agreement with Krugman, but I also think that he does not quite grasp the enormity of the transformation - to all of us - that is about to occur via smart automation (A.I.) and the 4th industrial revolution.