Anyone who has to write in any way extensively for a living will not find an iPad comfortable to use.Do you honestly think that most (not all) consumers actually need Windows or MacOS for day to day computing?
I accept that for some people the iPad will fulfil most - if not all of their needs. Likewise, others will switch between devices, using the iPad for some things and a computer with greater power (and a keyboard) for others.
But it will not replace the use of other devices, except for some people - chiefly, those for whom the consumption of content is what defines their online lives.
But for those who create content, the iPad is insufficient for their needs. But they will - or may use - an Pad for other things.