I think the usual counterargument to this is that those who don't contribute to society have emotional and social value to those who do. Ie the reason we don't euthanize allentally handicapped people is because of the emotional trauma (and related loss of capital) that would cause is more costly than the expense of keeping a mentally handicapped and non-earning person alive. Not that this counters what you're saying in your comment--i don't think you value economic modelling highly--but it just goes to show how many hoops you have to jump through to make it look like human lives are valuable outside of their capital in economic modelling. I wish something like that were more immediately obvious.If you're not capable of contributing to your society then what value do you truly have?
Your explanation is way easy to understand. In fact, I'd never really considered how valuable it is to keep a person alive for the sake of the family's emotional health. I just have my own personal values about it which don't necessarily generalize to a community as a whole.