If human slavery was a net-loss for countries then it wouldn't be historical popular. Be it building roads, railways, bridges, buildings, harvesting or picking fruits, those are not things people in general want to see prices increase. People who talk about illegal immigrants being a net-positive on the economy never talk about that aspect, in the same way that those being against illegal immigrants do not want to talk about increased costs. Even people who talk about human trafficking do not want to talk about human trafficking in construction or food production.

At one point the police even announced (as part of a political move in order to get more budget) that they would stop investigating construction places for human trafficking since just going to a single construction place would fill their work quota for that year, and thus everything else would had to be put at hold. Everyone who work in construction are fully aware of the open secret that a large part of all work is done by illegal workers that do not pay taxes (or minimum wages), do not get safety equipment, and is not limited by regulations that exist to protect workers. Sweden is far from unique in this aspect.