Its not convenient to carry around ANY sort of storage if you're into immediate gratification of looking at videos or pictures with your friends, which is what this is targeted for. "Remember that time we did X? Wait, let me find my keys, put in the storage, open the program etc". This is targeted at young people, not old patient tech nerds.
Anyone in that category who wants long term storage has many options there, and, just guessing, isn't using snapchat....
Just to re-emphasize, the cloud is about immediate gratification (keyword: IMMEDIATE not 1+ minutes from now). No one who uses snapchat enough to need these storage plans has enough patience to do what you suggest.