If the Federal Government can not or WILL not protect the borders of their own county they should be run off on a rail or worse. It is their main job.
I’m not trying to be contentious, but perhaps his policy is that which he perceives as the best way to enforce that which you see as the ‘prime directive’.
In other words, he can conceivably send in the “feds” to all 50 states and enforce the law- or- he can tell the states to deal with their immigration problem in compliance with federal law- or suffer the consequences.
It’s not an abdication of responsibility- it’s a method to fulfill it.