To me it's not even about cortisol and levels of harm; I just think in no other situation would I, if I had the option not to, leave someone I loved screaming and crying for me. I would go to my crying mum, partner, best friend, in the night and give them a cuddle and stay with them until they fell asleep if that's what they needed. So I don't understand why it becomes OK just because the person crying is a tiny helpless baby who doesn't yet have a way to verbalise why they are upset. Just as I don't understand why if my other half were to give me 'a little smack' when I didn't do as he told me, that would be a prosecutable offense, but if I do it to my tiny child that's just 'discipline'. Never mind the legality of it; why would we treat our children more harshly than we would other adults, when they are so much smaller and more dependent?
And yes I know we do things for and to children we wouldn't for other adults - change their nappies against their will, pick them up bodily and restrain them from doing something dangerous etc. But we do all these things to protect them from illness and disease; my little girl slept like hell as a child, still wakes up 1-2 times a night now as a 2 year old; but in the day she is bright as a button, talking in full sentences since 18 months, walking at 10 months. She is not being harmed by the lack of sleep, so there is no way I can kid myself leaving her to suffer alone would be for anybody's good but mine. When I had to leave her at nursery screaming and wailing for me, I had no choice. I had to go back to work and earn for our family. Every night when she cries out for me, I have a choice. And I choose to save her from being scared and sad.
Incidentally, when I initially left her at nursery for settling sessions, she would cry until I was out of sight and then instantly go to sleep (even if she had napped for an hour+ before we went there). At one point, the nursery staff became aware she was actually pretending to be asleep (she was peeping at them but when they looked at her or spoke to her she would shut her eyes and go floppy and unresponsive - this was at 11 months old).
I 100% believe this was a stress/fear response, which has only reinforced my belief that a baby learning to go back to sleep on waking through CIO/CC does not mean that the baby is not stressed - indeed that quite the opposite could be true in some cases.