My mother was an only child, so I imagine she was fully aware of how life can be for an "only" and she lost both her parents at a younger age than I did - her DF died when she was 18.
I worked with an only child, who had a parent who was an only child, and has an only child herself. Her DM recently died, on the other side of the world, and I've since been talking to her - she most certainly is not feeling the way you seem to believe she should be. You do seem to seach out people who insist on behaving in a way which, to put it bluntly, is not normal.
I did not suffer from depression when dealing with my elderly parents - who were divorced which makes it harder as you are dealing with people in separate living arrangements. Three of my friends also dealt with their aging parents - all had siblings, none of those siblings lived locally so they were doing most of it alone. Being part of a family doesn not mean that everyone pulls together.
The only way you are going to cope with what is to come is if you develop some resilience and deal with it in a calm and rational way. Looking for others who feel as you do is not healthy, and will not help you.
My parents would be appalled if they thought I was wallowing in grief, writing blogs about how sad and awful my life was, how I would never come to terms with losing them and that my life was now meaningless.