I love dogs. Owned some myself but I think once a dog goes blind and has debilitating arthritis, it's compassionate to put them down instead of allowing them to suffer longer then they have to just because we find it difficult to let go.
I've seen this way to often. It's understandable but selfish. I've seen families pay a boat load of money keeping an old pet alive. Money isn't the issue, it's the fact that the animals quality of life is so low that, as you said, it's more humane to end it's life.
A friend had an old dog who was on a variety of medications for heart issues and arthritis. The dog had to wear a diaper because it was incontinent. The dog rarely moved becuase of pain and spent most of the day sleeping. At this point, it's cruel to keep the dog alive. Eventually they realized this and had the dog put to sleep.