The entire economic argument for making (illegal) immigrants citizens is that they do the work that Americans won't do and that they contribute to the economy already. If you don't have to work anymore if you're a citizen, it completely destroys that argument for immigration. Also, the part where if UBI existed, those horrible-paying jobs would have to raise their wages to get anybody to do them, which would price immigrants out of the market because natives would then be incentivized to do them.