I think this subject should be treated with a certain amount of delicacy and balance. Obviously if you are talking to Novus Ordo people who think everyone goes to heaven especially them, they need to be confronted with stuff like this as a wake-up call.
But with people who actually believe in the Catholic Faith, who believe in Hell and believe one mortal sin will send them there, and believe that everything the Church says is mortally sinful is actually mortally sinful, this sort of topic can become frightening and discouraging if it is not balanced with considerations of the power of God's grace and His desire to help everyone save their soul.
Personally, I believe the percentage of people going to heaven has changed dramatically over the course of history. I think in the Middle Ages a much higher percentage of people were saved than in pagan times, and much more than today.
That being said, when I look at the traditional Catholics I know, the vast majority of them appear to me to be on the way of salvation, although only God sees the heart.
Since this information is hidden from us by God's will, I actually don't think about it that much and I don't think it's all that relevant to our salvation except as an incentive to try our utmost to be saved. Our souls are not graded on a curve; the number of other people who go to either place will not change the terms under which our souls will be judged anyway.
There are actually a lot of respectable theologians who have taught that they believe the majority of adult Catholics will be saved, due to the power of the sacraments and God's grace.
Fr. Faber wrote a
fascinating treatise on this question in his book "The Creator and the Creature" in which he compiles every opinion of every theologian he can get his hands on, and what they think about this. He specifically excludes any opinion about non-Catholics, but gives the results of his massive study on the question. I think he says a slight majority of theologians think the majority of Catholic adults will be lost, though if you include those dying in infancy then a majority of theologians believe a majority will be saved. He himself believes the great majority of adult Catholics will be saved, and spends the rest of the chapter explaining his reasons for thinking that.
His study is thorough, careful, prudent and wise. I highly recommend it to anyone interested in the question of what percentage of Catholics will be saved.