I’m quite tired of those grandiose declarations of what the iPad will achieve myself but I have to correct your history a bit.
We know of examples were new technology of delivering media killed old technology. CDs killed LPs, DVDs killed VHS tapes. (In the same way in which cars killed horse carriages – there are still some out there but they don’t really matter.) Other technologies fundamentally changed just how we use media. DVRs did that to TV.
So just why do we see certain change but not other kinds of change? My personal little theory is that certain modes of consuming media will pretty much always be useful, independent of the available technology.
Delivering just text will in certain situations probably always be cheaper than delivering visuals and audio (no need to produce expensive visuals or hire a speaker), I also think that it just might be more effective to take in certain information by reading rather than any other way. Consequently something like newspapers and books will always exist (with or without the text being printed on dead trees). You can make a very similar argument for Radio.
The big thing about computers has always been that they can potentially deliver everything. Just text. Just audio. Just visuals. Or all that in any combination you wish. So, sure, computers (and that includes the iPad) might just be able to kill newspapers, radio, TV, books. But just as we know them. There will probably always be devices which allow us to read text, listen to something or watch and listen to something.