An argument was made from Jacob’s instruction to his descendants to remain faithful to their religion, as if this necessarily barred acceptance of a later authentic prophetic mission or changed legal detail.
The inference does not follow. An instruction to remain faithful to God, revelation and righteous conduct need not make every earlier historical legal arrangement eternally immutable. Loyal obedience to a prophet’s true message is not equivalent to refusing later revelation merely because it differs in some particulars.
The mistake is to conflate fidelity to the religion’s core with perpetual retention of every prior institutional form.