This is a long term bet now on the success of BB10. The more and more sales that convert people to BB10 means less and less service revenues such that some time in the future the phone will be sold to all BB7 users and the transition is complete (18 months). At that point service revenues will be fraction of what they were. The stock should be pretty low by the point.
BUT Blackberry will have confirmation of say 50 million users on BB10 and could possibly attract some licensing deal for its OS and or get a buyout (24 months).
Unfortunately that may be at a share price of $12 per share by then.