These hypotheticals tend to accidentally reveal a disturbing worldview in the way they treat immigration as a natural phenomenon rather than people with agency of their own. It's dehumanizing.

For example, where does that 99,999,999th person sleep on the night they arrive in this country? What is their immediate plan? How and why did they come here? Your hypothetical has them almost emerging from the ether as an inherent problem rather than a person making an active decision to move to somewhere they think they will have a better life. If we stop providing them a better life, they'll stop coming. But the primary path to doing that is making life worse for everyone already here and none of us should want that.