People have to do what they have to do, and sleep deprivation is brutal, and I 'waited it out' and I'm glad I did but honestly, the extent to which I might have been a kinder, calmer, more fun mum if I hadn't been so fucking knackered might have outweighed the benefits of sleeping cuddled up all night long.
But, yeah, I do think it's pretty cruel to deliberately refuse to comfort a baby who is alone and frightened or sad or tired or cross. Maybe it's a necessary cruelty at times, but it's not a neutral thing.
I didn't drive, so have very few other examples to draw on of when I had no choice but to leave a baby sobbing. I have two children, so sometimes one had to wait, but mostly I had two arms and could cuddle a toddler who'd fallen over whilst still listening attentively to a monologuing older child.
There is of course no evidence of harm, but as I understand it, studies reliably show show only two-thirds of the population have a secure attachment pattern. I don't conclude from this that sleep training is the cause (and I don't conflate attachment theory and attachment parenting- kindness, empathy, consistency and responsiveness doesn't have to involve bedsharing or slings or bf on demand or whatever), but I do remember it whenever people shrug and say "oh, by the time they're five you can't tell, they're all happy, blah blah".
I think there has to be space to say "this is not a biologically normal way of parenting, but we don't have our village any more and it's really fucking hard and maternal mental health matters too". Maybe the place to say that is not where desperately tired parents are seeking advice on sleep training? I don't know, I don't read those threads.