Just out of curiosity would you develop software on an iPod? There are many things not suited for iPads. iOS makes a number of haptic compromises to work on a platform that is geared around portability. Being portable makes it a better choice for a lot of social tasks. But there are some things iOS devices are just not good for. I constantly see people using iOS devices giving their device far more attention then Mac OSX Macs. But this is also due in some part to the newness of this type of interface.
It just depends on the user. But it is a lot harder to have 2 different windows open at the same time on an iPad. Almost all my work on computers is done using multiple windows. I compare a lot of stuff, or look at something in one window and take notes in another. There are just too many things to where iOS isn't a good fit. But there are also a lot of things were iOS is a great fit. Both platforms have their advantages. It isn't that Mac aren't bought much any more. People still by a lot of Macs. But Apple sells an insane amount of iPhones and that dwarfs the Mac sales. By itself Mac sales is still enough to be a fortune 500 company.
The long term goals of the entire industry is slowly moving to a completely different kind of computing. Distributed computing is a very different kind of thing. With that the differences in the platforms become irrelevant. Well actually the different kinds of platforms would probably become 1 very different thing. Many companies are working on moving that direction right now. So it would depend on which market force caused a change first if Macs being done away with ever happened.
But as other things become more intelligent and more connectable the less value the computers we have today will be. I think the movement toward distributed computing with intelligent agents is far more like to take out Mac's then a push toward iOS. But at the same time such a move will almost certainly beef up the power of a cell phone because they are build to be portable. But this subject is to big to get into in a post. Whole books are written about this stuff, many, many books.
Ok so now can imagine iOS not replacing Macs