Yes, there are merits to the principle that you're stating. And, that is a viable sentiment under the circumstances, plus qualified with your correlated verses, .....it goes without saying that Paul is not a sadist, and that these people are not of the true Church.The text themselves explain the word and what it means. To 'cut off' is indeed to separate, to amputate. No issue. The problem is the carnal mind versus the spiritual mind.
Do any really think Paul meant a retaliation, to cut off the male reproductive, rather than to simply be 'amputated', 'separated from', 'cut off' from the body of Christ? [Matthew 5:29,30; Mark 9:43,45; Romans 12:4,5; 1 Corinthians 6:15, 12:12,18,20 KJB]? If so, I pity such, really.
However, let's read it 'their' way, anyway, from an OT perspective which deals with the Physical:
Ought not we think that the NT is the Spiritual, and that Paul understood this?
Deuteronomy 23:1 KJB - He that is wounded in the stones, or hath his privy member cut off, shall not enter into the congregation of the LORD.
1 Corinthians 15:46 KJB - Howbeit that was not first which is spiritual, but that which is natural; and afterward that which is spiritual.
John Gill [Baptist]:
Galatians 5:12 Notations - "... I would they were even cut off which trouble you. These words are a solemn wish of the apostle's with respect to the false teachers, or an imprecation of the judgment of God upon them; that they might be cut off out of the land of the living by the immediate hand of God, that they might do no more mischief to the churches of Christ: this he said not out of hatred to their persons, but from a concern for the glory of God, and the good of his people. The word here used answers to the Hebrew word קפח, and which is often made use of by the Jews in solemn imprecations; we read (o) of a righteous man, מקפח את בניו, "that cut off his children": the gloss upon it is, ..."
Adam Clarke [Methodist]:
Galatians 5:12 Notations - "... As the persons who were breeding all this confusion in the Churches of Galatia were members of that Church, the apostle appears to me to be simply expressing his desire that they might be cut off or excommunicated from the Church. Kypke has given an abundance of examples where the word is used to signify amputating; cutting off from society, office, etc.; excluding. ..."
Treasury of Scripture Knowldge CROSS-references:
"... cut: Gal_5:10, Gal_1:8-9; Gen_17:14; Exo_12:15, Exo_30:33; Lev_22:3; Jos_7:12, Jos_7:25; Joh_9:34; Act_5:5, Act_5:9; 1Co_5:13; Tit_3:10 ..."
B. W. Johnson:
Galatians 5:12 Notations - "... I would they were even cut off. These men are seeking to make the mark of bondage by cutting your flesh. I would that they would cut themselves off (see Revision). I take it that he means "Cut themselves off from the church so as to have nothing more to do with it." ..."
Jamieson, Fausset and Brown:
Galatians 5:12 Notations - "...were even cut off — even as they desire your foreskin to be cut off and cast away by circumcision, so would that they were even cut off from your communion, being worthless as a castaway foreskin (Gal_1:7, Gal_1:8; compare Php_3:2). The fathers, Jerome, Ambrose, Augustine, and Chrysostom, explain it, “Would that they would even cut themselves off,” that is, cut off not merely the foreskin, but the whole member: if circumcision be not enough for them, then let them have excision also; an outburst hardly suitable to the gravity of an apostle. But Gal_5:9, Gal_5:10 plainly point to excommunication as the judgment threatened against the troublers: and danger of the bad “leaven” spreading, as the reason for it. ..."
...but, in the context, he expresses himself in a sadistic way for the sake of analogy. i.e. they deserve emasculation, because they are mutilating the Church. Paul does not mean it literally, and he won't accept it as a means to either punish or correct them. He's just expressing, in a graphic manner, the harm that these Judiazers are imposing on the Church. His intent is to show an example, of doing whatever it takes to get them to stop. It's hyperbole for the sake of effect.
Ellicott's Commentary for English Readers
(12) I would they were even cut off.--The Authorised version is undoubtedly wrong here. The words may mean "cut themselves off," i.e., from your communion, but it seems far best to take the words, with all the ancient Greek interpreters and a large majority of modern commentators, including Dr. Lightfoot and Bishop Wordsworth, as referring to an extension of the rite of circumcision, such as the Galatians might see frequently practised by the priests of Cybele, whose worship had one of its most imporant centres in their country--I would they would even make themselves eunuchs. Let them carry their self-mutilation still further, and not stop at circumcision.