I find it striking and interesting that in the article where Aquinas officially addresses whether the Incarnation is fitting, his argument in favor of the fittingness of the Incarnation makes no mention of salvation. In other words, it's clear that the Incarnation would be fitting whether or not we sinned, though Aquinas is inclined to think that had we not sinned, the Incarnation would not have occurred. The focus on epistemic benefits is particularly interesting:
On the contrary, It would seem most fitting that by visible things the invisible things of God should be made known; for to this end was the whole world made, as is clear from the word of the Apostle (Romans 1:20): "For the invisible things of God . . . are clearly seen, being understood by the things that are made." But, as Damascene says (De Fide Orth. iii, 1), by the mystery of Incarnation are made known at once the goodness, the wisdom, the justice, and the power or might of God--"His goodness, for He did not despise the weakness of His own handiwork; His justice, since, on man's defeat, He caused the tyrant to be overcome by none other than man, and yet He did not snatch men forcibly from death; His wisdom, for He found a suitable discharge for a most heavy debt; His power, or infinite might, for there is nothing greater than for God to become incarnate . . ."Of course, in the next article, Aquinas does talk of the need for the Incarnation for our redemption. It's not an absolute need, but it is necessary for our redemption to be worked in a better and more fitting way (Aquinas compares it to the necessity of a horse for a journey--presumably, one could always walk, but a horse is better). In that article, Aquinas gives ten benefits for the sake of which the Incarnation was fitting in respect of our redemption. It is interesting that penal satisfaction is only one of the ten.
I answer that, To each things, that is befitting which belongs to it by reason of its very nature; thus, to reason befits man, since this belongs to him because he is of a rational nature. But the very nature of God is goodness, as is clear from Dionysius (Div. Nom. i). Hence, what belongs to the essence of goodness befits God. But it belongs to the essence of goodness to communicate itself to others, as is plain from Dionysius (Div. Nom. iv). Hence it belongs to the essence of the highest good to communicate itself in the highest manner to the creature, and this is brought about chiefly by "His so joining created nature to Himself that one Person is made up of these three--the Word, a soul and flesh," as Augustine says (De Trin. xiii). Hence it is manifest that it was fitting that God should become incarnate.
And then Aquinas adds: "And there are very many other advantages which accrued, above man's apprehension." How does he know? Is it just a confidence that God does many, many good things for us? Could one argue that the Incarnation of an infinite being must somehow bring infinitely many benefits, of which only finitely many will be understandable to us?