Speaking from the perspective of this side of the Atlantic, not all of them end up under a bridge. Few of them end up living the good life either, but if they work a menial job and can still send home remittances that’s a big step up for them.
I should be clear the research I’m thinking of is about drug cartels, so I’m not an expert either, but in what you’re describing it sounds like the smugglers get “a free lunch”, and there’s never a free lunch.
During the ice age, the Sahara was savanna, and it’s though to be possible it could go back due to climate change.
That would be a nice silver lining on a mostly very bad mistake, I guess.