A great question! Even those who are not fortunate enough to provide for long term care for their survivors can do much in preparation for this eventuality. Children with Down’s sadly do not often face this burden as their lifespan is not as long as those with autism. [medical care is an important factor in this over the life of the child in either case.]
Parents who have little in wealth can make make arrangements with their state Departments of Human Health and Welfare agencies. It is important to make sure the contact information and policies have not changed. High functioning autistic adults that can care for themselves, can and should, receive assistance with a regular visiting social worker. The can handle paperwork on behalf of the survivor and assist with unforeseen issues such as displacements due to property sale or damage, hospitalization and so on.
If a responsible survivor, say a brother or sister can also act as an advocate in conjunction with the state, so much the better. For those with means, Lawyers [remember Rain Man?] can provide valuable services and make decisions for the survivor, and if funding dries up, can collaborate with the state to achieve the same ends.
If the survivor is low functioning and has nothing in the way of a financial trust, then he or she often becomes a ward of the state. At this point what can happen, and the quality of their care depends vastly on the area you live in. Where you retire should factor this in if possible.
Hope this helps.
It is all about planning, research, and staying tuned to changes along the way. Hope this helps.