I agree that manufacturing jobs are now automated, so they aren't coming back. But your other points are not true - imagine the US halted all imports. If you halt all imports, companies either produce in the US or don't sell to US consumers - it doesn't matter what labor costs. With tariffs US companies don't have to compete with dirt-cheap international labor. Whether or not this is a good idea is debatable, but you can definitely use policies to promote domestic production.
If you want <exports>, with tariffs, US companies still have to compete with dirt-cheap international labor. This is what I meant by improving production but not production that <exports>.
Because when you export, you're once again selling on the global market. You must compete with all players. If you can't export, if your manufacturing machine is too expensive, or if your output is too non-unique, then you aren't creating the fertile atmosphere of Shenzhen. How is this going to increase export to generate more American wealth?
Having a production machine that makes things only for your nation and nowhere else is just a redistribution of money. Redistribution of money can be very healthy, but why not just do it directly without potentially wasteful intermediary processes?
Also, I mentioned the point of Shenzhen style abused labor manufacturing vs. robotic high tech labor, and that if manufacturing comes back to the US, it will likely be in the form of robotic manufacturing, and it's not obvious whether this manufacturing will have much room for the masses of unskilled workers.
