I also echo the sentiments expressed so far and am disappointed that discs by mail will be dropped. I'm a critic of the service. It leaves much to be desired. Its shortcomings have been well covered here before.
But I like the idea of getting a disc in the mail. I like old movies, serials and TV shows and that content is unlikely to be streamed. And I like having one disc at a time sitting in my DVD player so I can watch a chapter at a time when I have an opportunity. I don't want to have instant access to everything. It's like drinking from a fire hose. When I have a limited number of discs at my fingers, I'm going to enjoy them much more. The queue helps me organize and prioritize my viewing. Maybe it's an odd opinion, but I prefer to not having immediate access to everything, i.e. instant gratification.