I do have some sympathy with the idea of euthanasia for humans. However, it has to be a personal decision. I do not think we can impose our views on others.
And you really do not know what sort of life these elderly people have unless you are with them 24/7. My grandfather has really bad days where he doesnt know who he is and has no idea what is going on. However, he also has really good days when he recognises his family, enjoys singing songs to my mother's organ playing, enjoys his garden, goes out for lunch.
I would hate for someone else to make a decision on my life based on a short snapshot of time.