Of course Tim just carries around an iPad wherever he goes
He doesn't really do anything with it, right? Perhaps he checks email and messages and looks at his calendar, but what else does Tim Cook do on his iPad??
Professional people use real computers (and not just "devices" for "sharing" their "stuff") to do real work via spreadsheets, word processors, and presentation tools. Some will argue that not only can one just edit those types of files on an iPad, one can actually create them on it as well. I don't buy it. Perhaps some people can. For most people, however, I'll wager that creating those types of files is far easier with a keyboard, mouse, and large monitor(s). Still, the standard "office-type" applications are just for starters.
Then there are the heavy-duty applications for photo, audio, and video editing. Don't forget engineering design, modeling, analysis and simulation. Oh, wait, we omitted publishing, accounting/tax-preparation, and project planning. There are other vertical-market applications too numerous to list; a single such example involves the medical field--diagnostic tools, equipment-control, record-keeping, insurance case analysis, and many others. We don't have the space to get into software-development tools. I could go on.
So... no, real computers for real professionals aren't going anywhere. Tim is welcome to carry around his "devices" if he wishes, however.