> if Trump is impeached and (say) Hillary Clinton gets in. Would we see a collective sigh of relief and companies going back to the US?
Ermmm... what? That's exactly the wrong way round.
You clearly have never had any sort of contact with any sort of business, US or otherwise.
The US's tax regime had been driving companies and their money out of the US for many many years. Usually "under the hood" by US companies' figurehead-shingle staying in USA but most actual business being run under offshore companies. The money was kept offshore and in tax shelters because to bring it back or even to publicly reinvest it would trigger US taxes. ~US$2 trillion IIRC.
Nevertheless, many companies were still leaving in-toto, and the rate/$ize was increasing. Google "tax inversions". e.g., "inversions and other income-shifting techniques reduced Treasury revenues by as much as $111 billion in 2012." Obama's time, you'll notice.
Trump's early tax changes are worth reading in detail. They were excruciatingly dull. And surprisingly smart. Nearly all of that offshore money is now intra USA and invested, and a number of other less obvious tax redtape excruciations have been able to be literally wiped from the books because the subtle changes eliminated the loopholes and scheme-incentives which the redtape was imposed to block.
Key point: I have seen NO tax inversions initiated since Trump's tax changes.
To be clear: where companies were actually completely leaving the USA and in increasing numbers and size, the entire syndrome has vanished.
To be clear: you are exactly the wrong way round.
Trump is on a personal level a ratbag but on a professional level knows his stuff. One's attitude to him depends on whether you regard politics as a job with serious consequences, or as status theatre.