In other words, once you accept Paul's words as a defense of what we call "Calvinism", you really have no principled stopping point to letting that verse be used to silence any other objection. Why can't God condemn perfect beings to eternal torment? Who are you, O man, to answer back to God?
See especially Abraham's intercession on behalf of Sodom in Genesis 18, in which Abraham objects to God's intentions by saying "Far be it from You to do such a thing as this, to slay the righteous with the wicked, so that the righteous should be as the wicked; far be it from You! Shall not the Judge of all the earth do right?” (Gen. 18:25) Abraham objects to God's declared will by making an argument from God's revealed nature, and God agrees with him and relents. A Calvinist bystander armed with Romans 9:20 might have interrupted on God's behalf and rebuked Abraham.