As a person who has worked intensively with people like you are describing, they can be very lively and are in general a million percent happier than any of us. The only reason that someone would euthanize people with disabilities would be if they were too lazy to do the all the work that it takes to keep these people happy and healthy. It is a lot of work, but is also the most rewarding work in the world. The idea that any of the people I have worked with being euthanized is just as horrifying to me as a completely healthy person being euthanized. They have just as much to live for as we do, they just live their lives in a different way.