“Why our borders were left open so long will forever confound me.”
They were left open because thousands upon thousands of companies and millions upon millions of personal incomes depended on that travel and transport across borders. Take away those companies and their ability to provide an income to that many of your citizens - the income they must have to survive - and you collapse your economy. And your government then collapses when you make your people destitute and unable to maintain a basic standard of living.
It’s a Catch-22. There are only wrong answers. And when the panic sets in and you are forced to collapse your economy then, you arrive at the same place. Had the borders been closed before this, Trump would have been removed from office, one way or another, when the economy collapsed anyway.
We do not have to “guess” what it would look like. Look at countries who are proactive vs. those reactive. The proactive ones still have a functional economy and is not completely shut down.