Having grown up in the Rust Belt, it's a bit baffling to me that there are intelligent people out there who don't understand why it's a bad idea.
It's a race to the bottom. The jobs all go to countries where people are paid almost nothing to work 90-hour workweeks, safety and environmental concerns are not existent, and they have totalitarian political systems where anybody who complains about any of the above will be shot.
Said other countries have stronger economies which lifts their geopolitical influence and military power. You definitely don't want to give that to governments with those kinds of terrible value systems.
Part of our society's narrative is that anyone can join the middle class: "If you just work hard, you can get a good job, support a family and live a nice lifestyle." The companies formerly supporting that narrative moved their operations out of our country; those opportunities were never replaced. A narrative that binds our society together -- a fundamental part of the American soul -- is getting destroyed. Which is a big factor causing the terrible current political climate.
From the 1990's to today, a lot of Rust Belt places went from union blue to purple and then turned deep red in an instant in 2016 because somebody was finally acknowledging the problem.