I have an handicapped adult son and have been debating what to do in the long run. For those who can understand support them to the max.
Just from the people I know with adult children who CANNOT understand legal and financial issues there has to be some tunable form of guardianship that does not cost too much , does not require enormous paper work, and can last after the parent death.
Right now its self managed , Complex and expensive private trustees, or an unrevocable guardianship by an uncaring incompetent bureaucracy.