I quibble with one fact, and the rest is in interpretation. If there was a known author under consideration, I have never seen it attributed to Pope St. Gregory the Great, only to Thomas of Celano, and considering the late age of first this specific chant and second the genre of liturgical sequences, it can't have been written by Gregory.
It's a mark of these skill of these composers that they could insert it with varying degrees of subtlety. I found it highly appropriate that it is in the score for It's a Wonderful Life, since that film is about judgment and conversion in its own way, and also for the Lord of the Rings since Tolkien described the Ring as a metaphor for original sin, which of course leads us to sins for which we are judged. It also is in the scores for I Confess by Hitchcock and Disney's animated version of The Hunchback of Notre Dame.
Archbishop Bugnini has this to say in his book on the liturgical reforms:
They got rid of texts that smacked of a negative spirituality inherited from the Middle Ages. Thus they removed such familiar and even beloved texts as the "Libera Me, Domine", the "Dies Irae", and others that overemphasized judgment, fear, and despair. These they replaced with texts urging Christian hope and arguably giving more effective expression to faith in the resurrection.I think he distorts the texts of the traditional Requiem Mass, which gets its name from the Introit Requiem aeternam, which asks for the mercy of God to be shed on the person and that they might see God in the beatific vision. It's not all about judgment, although that is necessary. Not even the Dies Irae is like that. To cite one example, the Sequentia closes with a request to our dear loving and merciful Lord Jesus to grant them eternal rest, and that text Pie Jesu is frequently set as a motet or solo piece. (Andrew Lloyd Webber's is particularly moving, and if it weren't a solo I'd have it sung at my funeral.)
The funeral Mass is also an opportunity for us to come to God in our grief, recognizing that He is merciful (His greatest attribute being mercy) and that only in He can we be completely happy, so we can pray for the deceased's eternal happiness as well as our own temporal calming. I find that the mystery of the mercy of God is best unraveled in the traditional liturgy, and although I have only been so far to a low Mass said for the departed, I would say the Requiem Mass in particular is superior in this regard. Yes, in the newer form, the Paschal Mystery is more frequently referred to in its entirety (actually, I'd say the Prefaces talk about more of the work of Christ and the words "Paschal Mystery" make it into a few collects, but the anaphoras go no farther than the traditional anamnesis).
I also understand that in his mind and those of the reformers people associated the chant only with death. Perhaps that might be true in the Curia or in large cities with educated populations who had heard the various orchestral settings of the Requiem, such as Mozart's or Verdi's. Hence its banishment to the Liturgy of the Hours. But I dispute that people associated it with death like the composers did, even if the Sung Mass for the dead said on IV class days was common (the stipend was very low so it was a common practice). Why? Because the ordinary person heard it most often in its proper context, that of the Holy Mass said for the repose of the souls of the faithful departed. Bear in mind too the relative poverty of Europeans and Americans: peasantry (or its industrial equivalent) never really has disappeared from Europe, so I find the change to be elitist...well, I'll leave the criticism of elitism and arrogance to Fr. Hunwicke.
Besides, popular piety needs to inform the liturgy. Praying for the dead comes from doctrine and is expressed through that piety. It's hard to argue that before the Council, people prayed for the dead even if done in a perhaps superstitious or under-catechized way. Now people don't really pray for the dead, as a general trend. And that is awful, because how many poor souls are left in Purgatory? Even if your (insert loved one) is in Heaven, someone else's relative or friend might still be suffering for their sins but on the verge of Heaven! Fr. Ray Blake among many Catholic writers has paid close attention to this, and it bears new importance considering the Holy Father's appreciation of popular piety (see Evangelium Gaudium 122).