"Stop the boats" is merely a "Take back control" des nos jours.
By setting themselves an impossible task, the Tories believe that have hit the motherlode of polling. After all when it fails it can always be somebody elses fault and with a compliant press, you get to go again. That's why there are some people around who are still happy to vote Tory with the excuse they just haven't been able to get going yet. I mean 13 years isn't long, really.
(See also: War on Drugs. A cash cow for law enforcement (and those that administer law enforcement). In 51 years of this strategy, never once has a decrease in the amount of substances seized, or a reduction in people seeking treatment for addiction been a metric for success.)