It cannot be justified simply because "it is cheaper" for the government. It would be cheaper for the government to simply euthanize the mentally disabled rathe

It cannot be justified simply because "it is cheaper" for the government. It would be cheaper for the government to simply euthanize the mentally disabled rather than give them lifelong support and care that they can never repay, but we reject that as immoral. Historically, we've seen that making arguments like this on the basis of costs, numbers, and science leads to eugenics. It must be a moral argument, which has gotten harder to make in modern times as commonly-recognized sources of moral truth (religion, mostly) have been on the wane for some time.