Actually, looking this up, St. Thomas directly addressed the exultet. From the Catholic catechism :
But why did God not prevent the first man from sinning? St. Leo the Great responds, "Christ's inexpressible grace gave us blessings better than those the demon's envy had taken away."307 And St. Thomas Aquinas wrote, "There is nothing to prevent human nature's being raised up to something greater, even after sin; God permits evil in order to draw forth some greater good. Thus St. Paul says, 'Where sin increased, grace abounded all the more'; and the Exsultet sings, 'O happy fault,. . . which gained for us so great a Redeemer!'"From the original source:
Objection 3. Further, human nature has not been made more capable of grace by sin. But after sin it is capable of the grace of union, which is the greatest grace. Therefore, if man had not sinned, human nature would have been capable of this grace; nor would God have withheld from human nature any good it was capable of. Therefore, if man had not sinned, God would have become incarnate.Reply to Objection 3. A double capability may be remarked in human nature: one, in respect of the order of natural power, and this is always fulfilled by God, Who apportions to each according to its natural capability; the other in respect to the order of the Divine power, which all creatures implicitly obey; and the capability we speak of pertains to this. But God does not fulfil all such capabilities, otherwise God could do only what He has done in creatures, and this is false, as stated above (I, 105, 6). But there is no reason why human nature should not have been raised to something greater after sin. For God allows evils to happen in order to bring a greater good therefrom; hence it is written (Romans 5:20): "Where sin abounded, grace did more abound." Hence, too, in the blessing of the Paschal candle, we say: "O happy fault, that merited such and so great a Redeemer!"
Yancey’s information is not 100% accurate. Aquinas and Duns Scotus never knew one another, in fact, Duns Scotus would have been 8 years old when Aquinas died.
The fact is that Aquinas did not take a strong stance on the subject. In fact he wrote that “this is not a very important question” given the actual economy of grace and he himself admits that the opposite “opinion can also be called probable”. His stance in his Summa theologica is that “it is more probable” that “the work of the Incarnation was ordained by God as a remedy for sin, so that, if sin had not existed, the Incarnation would not have been.”
When Duns Scotus came on the scene and said that it was “absurd” to say that God’s greatest work in all creation (the Incarnation) was “occasioned” by sin, the disciples of Aquinas then took a strong, definitive stance based on what Aquinas called “more probable” (for the thomists or disciples of Aquinas it was not more probable, but quasi-infallible dogma—makes for good debates :)
First, notice that these lines speak of a Redeemer. Nowhere does it say that sin was necessary for the Incarnation, or that Adam’s fall occasioned the eternal predestination of Christ and his Incarnation. Simply put, if Adam had not sinned Christ would not have had to come as Redeemer and so the sin of Adam can be said to be necessary if Christ is to come as our Redeemer. ‘No sin, no Redeemer’; but it does not follow ‘no sin, no Incarnation.’
Another point to reflect upon is this, is the Roman Catholic liturgy inviting us to rejoice in Adam’s sin? “O happy fault”!?! Not hardly. This is Ambrose's poetic hymn praising God for the victory of Christ’s resurrection from the dead. He invites the earth to “rejoice”—obviously a poetic expression. He then says that this night was chosen by God “to see” the Risen Lord—poetry, since the night has no eyes to “see.” The Exultet is a poetic Easter proclamation of joy and victory which Thomas Aquinas and John Duns Scotus were both well aware of.was . Besides, to be happy and rejoice at another’s fall would be a sin against charity.
I don't think Roman Catholics are exulting in Adam’s sin and disobedience in the Exultet. But I'm sure that they are rejoicing with all Christians in the triumph of God over sin, Satan and eternal death through the Paschal mystery.
“O happy fault,” not because it caused the Incarnation, but because God in His mercy willed to remedy our woe in such a perfect way.
If you have a few minutes, you might find this interesting--it tackles the whole thing head on. Exultet
PRAISED BE JESUS CHRIST!