they're already doing it, so this isn't much of a 'what if'. the problem is that money is kept offshore and never spent, because it's at risk of getting taxed immediately upon repatriation.
my understanding is that is absolutely not true. Apple doesn't have $90B or whatever sitting in a bank vault in Ireland. That money goes to a bank in NY, and Apple has access to it and uses it, though that money can't be used freely as money which Apple has paid US taxes on. It is still part of the economy.
https://www.bloomberg.com/graphics/2016-apple-profits/