There are so few US-built ships (flag will be something more convenient anyway) that they could easily be used on other routes, even if production capacity were

There are so few US-built ships (flag will be something more convenient anyway) that they could easily be used on other routes, even if production capacity were increased several times over. But yeah, the most likely beneficiaries would be shipyards in third-party countries. Insofar as the goal is to disrupt Chinese market dominance, that would still be a win for this policy.