I like the idea of Cantus being very strong from the beginning.
It might also be that Cantus is strong and gets stronger as with age and use.

It's a bit like uranium.
There's great power stored inside and it gets more and more dangerous when you refine it and density grows. And at some point you get past a point where a chain reaction starts and there's a power surge and explosion. Thing is, even in nature it can happen.

So for Cantus, even if you do not do anything about it, it will eventually evolve to a point where it's too dangerous.
The only alternative is to try to control it, even if the risk is having children that become dangerous because they control it with bad intentions (hence elimination...)