Obviously when I say "work" I mean "work on a computer". No computer can dig ditches. If you want to quote narrow examples where iPad can be used that doesn't change the fact that people who need to do a lot of work on a computer, which is not "all cases", can't do it on an iPad, it's too slow and too clumsy. The iPad is a steep step down in productivity from a laptop or desktop for almost all serious work.
A lot of big corporations are converting to do most of their serious work on racks of servers in their data centers, or even in the "cloud", with tablets just as viewers or remote desktops. The IT and maintainance costs are vastly lower that way. It's not slow or a step down at all when an office worker can command some octo-core remote VMs from their iPad.