No, quite the contrary. It’s happening now (although to be honest, I feel that many who leave are not catechized properly). There are people out there who were catechized properly but let’s say they read about the priestly scandal and they think “by their fruits you shall know them” and then they think “maybe everything I learned from the Church is a lie”. They have an experience or acquire new knowledge that convinces them that what they learned was wrong. I’m not saying it is right that they leave, but that it is possible to change your mind about what you were taught. This is not, in my opinion, what the Church is talking about in that paragraph. I think that paragraph is specifically talking about a willful disobedience to God.
The Church teaches us that there is such a sin that severs us from our relationship from God. If we commit a mortal sin and do not repent before we die we could not, should not, will not be saved. However, there are three requirements in order for a sin to be mortal. It must be committed with full knowledge, complete consent and it must be a grave matter. The paragraph you cited describes such a situation. The CCC is not implying that one cannot repent of this sin. So, when it says they could not be saved it simply means they could not be saved if they die without repentance.