This is a huge hurdle. There is actually some chip/silicon fab sites that never left (only scaled down considerably from their peak). A family member has worked at one for many years as an engineer/manager. When he tells me stories about the insane complexity of their process engineering it really hits home. Impurities at the nanometer scale can ruin 10 million dollar batches of silicon. Nasty chemicals are piped everywhere at the plant - the kind that explode if touching oxygen in the atmosphere or acids that dissolve bone before they noticeably affect your skin.

Running such a production is difficult.

The US isn't starting from scratch. And presumably the total cost includes all the R&D cycles. TSMC was started by a Texas Instruments employee.

But, if we assume they were, just double the cost as a sign-on bonus for all the engineers and poach away.

This is problem that can be solved with money, and not a lot of it in the grand scheme of things.