Some people are able to overcome major losses, some are not. Not just the loss of a child by any manner, but a significant other due to breakup or death, or the loss of income, or the loss of a home, or the loss of one's country due to war and having to become a refugee.
Major loss happens to everyone if they live long enough, and some losses are harder than others to recover from. Some do not have the inner resources and/or outer support to overcome such losses, but we can't be responsible for what others have nor lack, it's impossible to own that.
It's a dual challenge for the one considering ctb: knowing the pain others will experience, and considering their resources, abilities, and support, yet also knowing one's own situation, and not having the resources, abilities, and support to go on in order to not harm others. If we can't overcome, we know that a consequence beyond our control is that others will have to figure out if they can overcome the loss.
It can be a never-ending cycle: I hurt/have no hope, my best/only option is to end it, and when I end it, another will hurt, and it hurts to know that, and I feel responsible for causing that pain, so I can neither bear what is nor the knowledge that another may not be able to bear what comes after.
I think suicide in many circumstances is a form of self-preservation, and that's maybe one place where the amorphous and condemning idea of selfishness comes in. But to survive can be too high of a cost, and it gets twisted around that others have to pay that cost, which is gaslighting in a way, that is, telling a different story than what is actually happening. The costs are separate but related, just as the child is separate from but related to the parent.
Major losses happen in life, and the suicide of a loved one can be one of those losses. If we don't have the foundations to support our own lives, we cannot take on the burden of responsibility for others' well-being, the foundation simply can't support it. If they also do not have the foundation to manage the loss, we cannot own that. It's already all too much. Yes, humans have social responsibilities to one another, but if we cannot manage our own lives, we certainly cannot take on the responsibilities for others' lives, most especially if they are adults, which parents are. As many have said on this forum, if you really want to save someone, then you should perhaps have to take on the responsibility for their life and their problems. If you can't own that, then let them do as they choose with what is theirs to own.