If we were smart we would ban people from leaving and entering the US, instead of waiting for another country to ban travelers from here.
However, a hypothesis that international travel is the major enabler reduces this to more of a 1 in 20 year event (SARS, MERS, Ebola, etc, etc all were near misses that were a little less slippery). Swine flu was also pretty bad.
We might be able to afford this once a century. We can't afford to do this once every 20 years. It is quite possible that it makes economic sense to permanently cripple international travel by introducing mandatory 2-week quarantines on arrival or somesuch.
Prevent stuff going bad at the root cause rather than trying to limit its effects.
Also you can't test for every disease, especially new ones. The best you could do is a several week quarantine on all incoming travelers I would think?
With all we know about our citizens I would love to see a wordlwide pandamic prevention travel policy that isn't activated but rather part of the fabric of travel. It could be automatic to start screening and potentially banning travel from somewhere reporting an outbreak.
Instead we had to wait for all the dominoes to fall and the lethargic arm of bureaucracy to move into action.
With a more rapid and precise response we might be able to avoid huge shutdowns like we have been seeing.