I don't see why would need them. Like I said we have plenty of room. It's logistically impossible for an overwhelming amount of the world's population to migrate.
It's the processing that causes all the problems. Green cards, temporary visas, tourist visas, blah blah blah. All just meaningless classifications so some government bureaucrat can justify his paycheck. If you want in and you pass a background check you get in. If you come here "illegally", aka jump a fence, and you get caught then you're subject to the same quick background check and sent on your way if you pass or detained if you don't. If these Central American/Mexican immigrants didn't have to go through years of bullshit to immigrate legally then they wouldn't need to hop a fence. Just walk up to any entry point and you're good. The vast majority of people skirting legal checkpoints then would be doing so for probable criminal reasons and border patrol wouldn't constantly be having to separate the wheat from the chaff.