Helena shows the most disdain we see for the "innies", expressing firm conviction that they are not people. James expresses this too.

By EOS2, the lore is basically spelled out: Kier started Lumon with the desire of curing humanity of pain, after having had seen the fucking miracle of ether. Seriously, they make monuments of this stuff.

Imagine being one of the first people to get a leg amputation, and to get a swig of ether instead of a swig of whiskey. You'd worship ether religiously. And so did Kier, as a Civil War medic.

But ether use is bad for your brain, and Kier's introduction of ether to cure the woes of factory workers was disastrous. You can't save people from pain by taking a chemical hammer to their brain.

Introducing the severance procedure. Suddenly, you CAN save people from pain! It's way better than ether. There are no negative side effects, it won't hurt the baby during childbirth, etc.

It's a fucking miracle, just like ether. There are no side effects, no ethical proble- aw, fuck.

The ethical problem with severance is that it creates a new conscious. The only way severance serves Kier's original goals of liberation from pain is if the new consciousness is not a person.

And so, that becomes a message that is pushed through the infrastructure of Kier's religion and mythology, and it becomes a necessary belief, because... Well, to believe otherwise, you'd not just have to accept that your miracle solution is a bust, but also that you're an evil slaver.

(There are connections to the US abolition movement but that's a whole another post)

TLDR: Severance is ether for a new generation, baby! But that only works if you really, truly believe that the new severed personality is not a person.