Laymen have basically been saying this for years while being ignored by the ruling elites but economists have been slowly starting to get on board.
Even post Trump I suspect that the global attitude toward free trade will become much more nuanced than it had been over the past 20+ years.
The trade shocks should not be handled with tariffs or restricting trade. It's domestic policy issue.
So of course the worker who over-invested in job A and was compensated based on that over-investment is going to be worse off in job B when he/she has under-invested in the skills needed for job B.
The fields that are best for workers today are those that have built in required continuing education, or fields in which rapid change is widely acknowledged and understood by the workforce (software engineering, etc.)
This is an important point, and I think plays into the question about tariffs (which, to be clear, I think are probably net negative).
The conventional argument is that free trade is a Kaldor Hicks [0] improvement, not a Pareto improvement. KH means that the sum of benefits and costs is positive, and it would be possible (in principle) to tax the beneficiaries and pay off the people harmed and create a synthetic Pareto outcome.
You could view tariffs as a clumsy attempt to apply the Kaldor-Hicks concept. I personally think there are much better ways, but I think the argument is at least plausible.
[0] https://en.wikipedia.org/wiki/Kaldor%E2%80%93Hicks_efficienc...
We've seen a lot of innovation in the area of free trade, commerce, etc., but virtually no innovation in education, on the job training, and other areas where policy can help make the economy stronger overall.
So in my view, tariffs are used in a way that is meant to undo the damage from decades of neglect of our educational and skills training infrastructure.
This doesn't call for a more nuanced view on trade (freer is always better), it is a call for us to begin to think rationally about job training.
Whatever job anyone has today, whatever product is demanded today, etc., will not necessarily be the same tomorrow. When we pretend otherwise -- such as the fiction that first world nations should manufacture steel -- we invite all sorts of bad policy when what we really needed was honest understanding that economies change.
Keep in mind that most moral progress over time is not the result of rational people convincing others of something that was previously opaque, it is due simply to economic progress. In other words, moral progress is a luxury we buy with prosperity.
Where we lag in this area most today is that we fail to understand that we must invest heavily in job training, education, entrepreneurship, and other areas that create new life when industries change, move abroad, or shrink.
It so happens that in US politics there are some influential electoral districts that contain steel mills and which have suffered from economic globalization. Sure the easiest way to placate those districts is to introduce tariffs or use other means to offer welfare that prevent those impacted from adapting.
Most of the buildings that house startups in urban parts of the US were once used for manufacturing or meat packing. This is a good thing in the medium term.
But creating job training or investing in new industries to replace old ones is not economics, it's social policy. We should not make the mistake of confusing it with economics.
The grocer example isn't really applicable. We both use the same currency so the grocer can use the local currency I pay for groceries to buy goods elsewhere or pay their employees. With foreign trade you have different currencies and a current account, the trade equation isn't as straightforward. An accurate analogy would be buying groceries via a barter system where I'd exchange good or services for groceries or a promise for future goods or services (eg, "luke-bucks") in exchange for groceries.