Strawman argument.
No way we can deport 11 million people.
We can't catch every murderer, rapist, or thief either. Shall we just throw up our hands, then, and announce that even though they've violated some laws, they do make valuable contributions to the economy, pay taxes, etc., and therefore, we need to come up with a program to bring them into society rather than trying in vain to hunt them down "like criminals"?
The logic is the same -- and if it's logical logic, it will hold. What stands for one, will stand for the other -- and what fails for one will fail for the other. Examining the logic of a situation in this manner is a standard technique for testing the validity of the logic.
And obviously this "logic" fails the sniff test.
The "there are so many of them that we have to make them legal" argument is logically bankrupt.
"Can't deport them all"? Well, then deport the ones you can deport. (And then watch the rest scurry back home as fast as their feet can take 'em, lest they be the next one's to end up in a "Sheriff Joe"-style abode!)
It's no different from prosecuting purse-snatchers. Sure, you "can't catch them all" -- but, you DO prosecute the ones you CAN catch.
The "can't deport them ALL" nonsense is disingenous strawman term-reframing, masquerading as defeatism.
If not for the fact that some purse-snatchers -- and rapists, killers, thieves, etc. -- are prosecuted, society would enter a downward spiral into utter chaos.
It is only the risk or apprehension that keeps most criminals, and would-be criminals in check.
Eliminate the risk, and welcome to "Lord of The Flies" style "society."
The exact same principle applies here. Of course we "can't catch them all." Hell, we couldn't "catch them all" even if there were only a few hundred of 'em! "Can't catch them all"/"Can't deport them all" is NOT the issue!
By essentially not apprehending any of them (which is the situation now, when those who are caught are set loose and given a "please report back, please" card), we are laying out the Welcome Mat. It's an open invitation.
We don't need to fine-tune the situation. We need a 180 degree course reversal. Anything else is smoke and mirrors.