They're framed with that assumption because of the other articles that call it "the future of computing". If something's the future of computing, it's supposed to supplant the present of computing, right?
Sailboats are luxury items, dude. People aren't forced to use sailboats. In fact, instead of 100% of ships being sailboats, I'd gander only 5% or less are sailboats.
5% of the sailboat market is pretty significant - as is 5% of the computer market. In fact, iirc, Apple has been making solid profits from round about 5% of the laptop market.
Anyway, I was merely illustrating that "the future of" does not mean "the sole replacement of".
It certainly simplifies things to say x will replace y. I suspect that with the ipad, its more of the beginnings of a new facet of computing.
For me, as a developer, the ipad certainly won't be much more than a diversion or a target for development. For people like my wife, though, I think it will definitely have an impact. Many people simply want a content consumption device (web, media, etc), and I think the ipad is a very good device for consuming most content.
Don't forget the cost. If its the future, its a must have thing. But its also expensive, meaning a lot of people can have only one and its a choice between the future of computing and everything else.