Logistical reality matters more here than philosophy. Even if birthright citizenship was dumb and we didn't want it, what are we going to do with those people? We can't deport them, no other country wants them either, especially since they're not from any country except for this one. We could pay another country to keep them as prisoners, that raises obvious ethical issues as well as economic ones, isn't it better to roll the dice that they might contribute to GDP instead of guaranteeing they're a net negative on it? Worst case scenario, they end up being bad citizens and then we can pay ourselves or someone else to jail them.