In light of the argument that was made from scripture, Jacob declares the current word of God. The principle is that what God intends for a current people is not dependent upon what was required or allowed for a previous time and place.
The command is clear. The current law is that there be only a single wife. There is no other legal allowance, either for other wives or for concubines. With that declaration, Jacob reminds the people that the promise of the land is that they are preserved upon the condition of living the commandments. The reverse side of that promise is destruction, and that is what Jacob emphasizes when he indicates that the land would be cursed for their sakes.
That leaves a small problem in that there is still the scriptural justification that is contrary to the current commandment. Jacob declares that there are times when Jehovah’s commands to his people are conditional for the times and circumstances of his people. It is possible that Jehovah could command, or allow, multiple wives. However, that is not the current case. God’s word today takes precedence over God’s word to a different people and time.