Ensoulment-at-conception is an old belief but it was never the consesus position. See
catholiceducation.org/articles/medical_ethics/me0116.htm
Observe that CCC #2270 says nothing about souls. In fact, if you read Catholic doctrine on embryos, souls are never mentioned. Instead, the argument is that since zygote progressively develops into an adult, there is no obvious boundary between a proto-human and human, and so there is no ontological difference between the two. Ensoulment-at-conception is now indeed widely believed, but it is as a sort of ex post rationalization: if adults are ensouled, and adults are ontologically the same as embryos, then human embryos must be ensouled, because they are, well, human. And they are human because they have human DNA. Thus human DNA = soul. Soul is infused when the sperm fertilizes ovum and new human DNA is created.
Except…
There is more than one way to make a human. For example, in cloning, you take an ovum, remove its DNA, then you take somatic DNA from an adult, and put that into the ovum. There is no magical moment when new DNA is created. Or, you can synthesize the human DNA sequence in the lab and put that into an ovum (bonus points for also synthesizing the ovum). Again, no fertilization to speak of, so no ensoulment. Or just try human-animal hybrids and designer genes.
While you can rationalize that ensoulment somehow happens in these scenarios, you can also rationalize that ensoulment doesn’t happen, so the end product is not ensouled, ergo not human. In right (or wrong, depending on your POV) socio-econo-political conditions the second argument may prevail, opening the door to mass-production of “unensouled” clones. For what happens next, see
Blade Runner or
Cloud Atlas.
If you say that it’s impossible for millions of Catholics to fall into such gross moral error well… It should have been impossible for millions of German and Austrian Catholics to fall into the gross moral error of supporting the Nazi party and perpetrating the Holocaust… But they did, because Nazi ideology was rooted in some elements of Catholicism… One man only saw through this – Franz Jägerstätter – and he died abandoned by his Church.
Ensoulment-at-conception today → Holocaust of the clones in 50 (100? 200?) years.