I don't think I ever lost an adult shrimp to planaria so I doubt you will notice your population decline. I always had several berried shrimp in my tanks when I had planaria but rarely did any babies make it to adulthood. I think the toxins the planaria carry are much more dangerous to smaller shrimp than adults.
I used to occasionally see adult shrimp "dragging" one of their legs with sand stuck to it. It is my theory that the slime secreted by the planaria somehow sticks the sand to the shrimp's legs. While this doesn't seem to be an issue for adult shrimp, babies probably get stuck to the sand and cannot move. They either then die of starvation or the planaria get them after the lights go out. Again, just a theory...
I used to occasionally see adult shrimp "dragging" one of their legs with sand stuck to it. It is my theory that the slime secreted by the planaria somehow sticks the sand to the shrimp's legs. While this doesn't seem to be an issue for adult shrimp, babies probably get stuck to the sand and cannot move. They either then die of starvation or the planaria get them after the lights go out. Again, just a theory...