We must not misunderstand Paul’s question here. It has a very precise place in his argument at this point.
God forbid that he should put in place a scheme of salvation that was unnecessary, and which was so wasteful: sacrificing the life of his only begotten Son for what could be achieved without his coming. The Father did put his Son through such agony and shame when in fact we could obtain righteousness by obedience to the law. God does not do for us what we are perfectly able to do for ourselves. If he did, a cynic would object that God was claiming credit for an act that could be equalled by mere human effort. Nor would God’s people be able to thank him sincerely, for they would know that they could have obtained righteousness without him, and their praise would at best be half-hearted. Instead he waits until we are helpless and there is nothing that we can do to save ourselves before he steps in to deliver us.