That's a bad assumption.
By the time fish have clearly identifiable symptoms, they've got so many worms inside of them that they're already quite weak. And then quite suddenly, the worms die, and must ALL be successfully passed. This is a highly traumatic experience, and a lot can go wrong, resulting in fish death. They may either fail to detach and then rot, or clog up the intestines entirely. The callamanus treatment document linked earlier in this thread makes brief note of this:
"Dead worms inside the fish may cause future mortality."
But more documentation is available elsewhere.
I lost a few fish when I initially treated for widespread callamanus infestation in my tanks. But during the follow-up treatment to knock out any new worms from eggs, no losses. And also no losses during the many, many times I've used it subsequently, using it as part of my standard QT protocol.
I'm not basing that statement off an assumption. I'm basing it off of evaluation and observation of the fish. There is a reason this stuff kills the worms, snails, and crustaceans in a tank. Ultimately, any medication is a poison.
I have no doubt that what you are saying is correct, but not the only variable in the equation. The fish become noticeably sluggish while the tank is dosed. Yes, they do stop eating, but I noticed the sluggishness within an hour of dosing. If you look up the medication, you will see just how potent it is
That post was a follow up post of wkndracers posts of know what you are putting into your tank and make sure you follow the directions. There is an obvious reason to follow charles' instructions which was exactly what I had observed with my fish. If you overdose, it WILL KILL the fish