Why would you consider Limbo to be a punishment?
Aquinas taught that it was a place of natural happiness.
Granted, heaven is a place of supernatural happiness, but we have no claim on Heaven, it is a free gift of God.
So not being given a gift we don’t deserve in the first place, but otherwise living in eternal happiness greater than we could ever experience in mortal life hardly seems like a punishment.
Brendan, thanks for your contribution!
We who realize why the Catholic Church can’t, and won’t, “abolish” limbo shouldn’t hesitate to grant that limbo is, in fact, a punishment. The theology of limbo is, it seems to me, multifaceted and somewhat complicated–but its facets and complications are hardly beyond the ken of the average Catholic.
In one sense, limbo *is *a punishment: the condition of having original sin, as Pope Innocent III taught, is penalized by deprivation of the supernatural happiness of the beatific vision. Nonetheless, in another sense, limbo is simultaneously a wonderful gift, since unmerited and unending natural happiness can only be described as a gift.
A rough analogy: the runner who comes in second is penalized by being forced to accept a less impressive trophy or medal because he lacks the ability of the runner who comes in first. Still, the second-place runner may–and should–rejoice in that lesser gift. In one sense, the second-place award is his punishment for the training he didn’t, or couldn’t, do. But, in anothers sense, it’s his reward for the training he did do–or merely for the fact that there were no other runners in his age category!

(I’m a guy who took up running late in life.)
Limbo is the second-place award.
It appears to me that the Magisterium, in a change of emphasis and terminology (not a true change in doctrine), has begun, with section 1261 of the
Catechism of the Catholic Church, to underscore the understanding of limbo as a gift of eternal happiness emanating from God’s love and mercy.
But this shift of emphasis cannot be described as “abolishing” limbo, and it doesn’t retract what the Church said in the Middle Ages about limbo’s punitive aspect. Accordingly, Catholic doctrine on the fate of unbaptized children and on original sin (which the new catechism calls “an essential truth of the faith” [section 388]) remains intact and coherent. Saying that those children all go to heaven, however, would unravel the whole fabric of Catholic doctrine. That’s why we can’t say that.
Thanks for mentioning St. Thomas! We should all read what St. Thomas wrote about limbo.
Keep and spread the Faith.