Oh, the argument was a stupid argument, but your refutation was equally so.
+1
The Constitution actually contemplated the end of slavery by kicking the issue down the road to 1808.
Many of the framers (though hypocritically slave-owners themselves) voiced opposition:
"It is much to be wished that slavery may be abolished. The honour of the States, as well as justice and humanity, in my opinion, loudly call upon them to emancipate these unhappy people. To contend for our own liberty, and to deny that blessing to others, involves an inconsistency not to be excused." - John Jay
"All good men wish the entire abolition of slavery, as soon as it can take place with safety to the public, and for the lasting good of the present wretched race of slaves." - Oliver Ellsworth
"There must doubtless be an unhappy influence on the manners of our people produced by the existence of slavery among us. The whole commerce between master and slave is a perpetual exercise of the most boisterous passions, the most unremitting despotism on the one part, and degrading submissions on the other. Our children see this, and learn to imitate it; for man is an imitative animal. This quality is the germ of all education in him." - Thomas Jefferson
"Slavery is such an atrocious debasement of human nature, that its very extirpation, if not performed with solicitous care, may sometimes open a source of serious evils." - Benjamin Franklin