Certain professions that don't prioritize specific softwares not available in iOS form can use iPads as laptop/computer replacement. I'm an engineering student, and I know a lot of people in engineering/science field (including med, pharmacy, etc.) Certain aspects of their works CAN be done on tablets. I've noticed gradual increase in almost professional engineering apps (not just from App Store, but also independently designed apps for company uses). But at this point they are more of secondary/assistant tools than main tool.
However, It's true most computers in workplaces are used for word-processing, PDF documents, emails, scheduling, etc. But, like I've said before, getting these tasks done in tablets take more time than on actual computers. Switching between apps is a hassle since there is no "desktop" environment where multiple apps can be opened side-by-side. And the way we interact with the tablet (touch-screen) isn't perfectly optimized for those multi-tasking. Well, at least for us who are used to working on computers. Who knows? Today's kids type their reports, essays, assignments, etc on iPhones/iPod Touches, and when these kids grow up they might prefer working on tablets over actual computers.
For casual users who use their computers for, well, things like browsing, streaming, emails, storing pictures, etc, tablets is an alternative to laptop, not a "replacement." Tablets handle certain things better than laptops while laptops handle certain things better than tablets. It's not perfectly black-and-white. I prefer certain aspects of viewing pictures on tablets (overall interaction, zooming, etc) while I also prefer certain aspects of viewing pictures on computers (picture info, light editing, folder organization, etc).
But most importantly, people have to be willing to re-learn and re-train themselves to use tablets as efficiently as they use their computers. People aren't going to throw away two/three-decades worth of what they are comfortably used to just to switch to tablets unless tablets offer compelling advantages. Yes, tablets are light, compact, offers tons of apps, can have LTE service, etc. But general population hasn't ditched computers for those capabilities, yet.