What planet are you living on? THEY'RE REQUIRED TO RETURN HOME UNDER EXISTING LAW, yet they're not doing it. What makes you think they'd return home under this new law?
There will be two key differences:
1. The guest worker will have been registered and fingerprinted before entering the country, and will have a record of his whereabouts.
In case it escaped your notice, we don't have that with the current situation. It will make it much easier to catch those who are breaking the law.
2. The employer will notify the government when the worker is no longer in his employ.
Again, we don't have that with the current situation.
The illegal alien problem was a hell of a lot more manageable before 1965. Returning to the pre-1965 standard would be the best solution, IMNHO.