2 things:

1) When you give blood, your body can make more blood.

If a person is given a soul, it can’t return to the life stream to allow a new individual to be born. Unless souls are infinite, there will be a point where souls run out and new babies can’t be born. We can infer souls are not infinite because Sphene plans to harvest every last ounce of life-energy until there is none left, then move on to other shards. If it was sustainable to “harvest life force” in any way, the AI would have thought of making a gigantic fish farm or something.

2) The extraction process turns the persons memories into an endless.

They will live (essentially) in a never ending twilight zone of a theme park for eternity. That is not a peaceful exit, and it might even be desired for a short period, but all indication is that they are in a sort of purgatory.