They don't know that they will get conservative votes no matter what.
What they do know is that when you run the numbers, the disgruntled faction is smaller than the disgruntled faction would be if they did exactly what the disgruntled conservatives wanted.
It's really not rocket science.
In your dreams. If the GOP acted to secure the border, they'd not only pick up the "disgruntled faction", but would take a lot of swing votes away from the Democrats also.