The problem with this is that "absolutely anybody in any country" can be a future potential tax payer, there are way more people willing to migrate into the US

The problem with this is that "absolutely anybody in any country" can be a future potential tax payer, there are way more people willing to migrate into the US than there is economic capacity to integrate them all. Not to say that an excessive rate of migration, especially concentrated to certain hotspots, can have other political backlashes - see the Trump era. A financial threshold is a nice way to limit that influx to manageable numbers, while guaranteeing they can mostly take care of themselves and become productive taxpayers and without exposing them to employer exploitation. The actual amount is debatable of course, it could be 2x the national average salary, 1x, 3x... or it could be dynamically allocated to fulfill an intake quota.