The ancient issues killed children and the weak
We adapted by having more children, with the understanding that many wouldn't make it
Would you prefer to live like this? In this more brutish world
From an evolutionary perspective, it might be the only way the human species survives and thrives.
Not much fun being the one naturally selected out.
You don't have to give worms to everybody. If healthy adults would be better off with worms (and that's a big if) you could infect just adults. And then give them worming pills in late middle age when their immune systems are weakening.