I'm sure the labor forces in the Philippines, Vietnam and Indonesia would be happy to work for Japanese businesses that have evacuated China and moved elsewhere, replacing the workers in China that have been let go as a result. Pick up the slack means to do the work that someone else has stopped doing. Since Chinese workers aren't doing the work, it must be done by someone else - they must 'pick up the slack' Slack here comes from a slack (loose) rope, which must be pulled tight.