I think yours is an excellent and thoughtful post and I agree with what you are saying.
I'm not sure about your conclusions here though and I think you might be underestimating the degree of change that is coming, I think it goes beyond our current understanding of productivity.
It's one thing for Cory to point out that middle class jobs are going to face significant challenges from automation, but the idea that these "high middle" jobs are somehow safe is wholly mistaken.
All the people displaced by automation are left competing for the "high middle" jobs, so standards required for those jobs will rise dramatically, and much of the work force across the board will simply be displaced, unable to find work, because human labor will no longer be the major component of productivity.
This will be a social shift the likes of which has never been seen before.
Those left displaced by automation won't have money, so they won't be buying the stuff that's produced, participating in the economy. So how are the people in the "upper middle" jobs getting paid? Automation will be encroaching around even those safe jobs.
A significant proportion of healthy adults will be unemployed within decades from now, and they will either be granted basic housing and living conditions by some yet to be devised economic mechanism, but one that is by definition socialist, or they will be a desperate army living in poverty and kept in check by jack booted thugs until they revolt.
I'm not seeing where the useful work you speak of is going to come from.