Them moving somewhere else is an easy fix. Just put an exit tax on the ultra wealthy.
Some of the mechanisms are loopholes, that might be closed l. But many start to interact with international business regulations that exist for considered reasons, and are harder to change even if it is serving as a loophole.
You end up with only the small wealth (one lifetime as a skilled professional) group getting caught
Nobody needs these billionaires; we can create new billionaires and new products. They think they bring some sort of ultra speciality but in reality they are doing something millions want to do and their monopolistic success is preventing others from succeeding; knocking these giants down makes rooms for new businesses and products. This is the entire thrust of a capitalistic economy.