I just dont believe FM/AM will go away in my lifetime at least. 100 years from now maybe?
I'm thinking that the year 2030 might be a soft target date for a transition to all digital broadcasts, at least on what is now the FM band. Analog FM is a spectrum hog. You could have much more diversity of programming with digital, and since it would be pure digital, with appropriate power levels, the current signal issues with IBOC would be eliminated.
Of course, it is always possible that advancements in mobile internet technology--that is as portable as current radios--could make the traditional broadcast model from a central transmitter--obsolete.
I think the medium-wave band (AM) is probably going to be toast, regardless of any digital technology. Nighttime skywave issues, the limitations of digital at such low frequencies, plus the often sizeable transmission facilities (in an era of urban sprawl) are going to work against it. Other platforms will make more sense from a business and programming perspective.