As a response they move their wealth overseas and renounce their citizenship if necessary. What strategy from the banana republic playbook would you like to see the State employ next?
It is frankley pathetic to suggest you can't tax rich people or they will go abroad.
Obviously you can tax income and put the mandate of enforcement on companies that are paying individuals. You cannot delegate the taxation of pre-existing wealth to companies though and deputize every firm as an IRS agent.