"and run all the important apps"
This is why laptops (and yes, even desktops) will remain relevant far longer than another "replacement cycle" - content creation apps on smartphones and tablets are complete and utter garbage.
Ever try scanning 10,000 pages into a tablet, OCR'ing them, keyword searching, cropping relevant portions, and inserting those portions into a slide show or video presentation? I just listed 6 things you aren't going to do effectively on a smartphone or tablet anytime in the next couple of replacement cycles, because the apps simply don't exist at all or don't have anywhere near the necessary functionality. And yet, those functions are necessary for most law offices - so you've just taken the entire legal profession off the map.
What about design engineering? Architecture? Accounting? Etc, etc, etc. The apps don't exist, or the ones that do are such complete and utter crap as to make working on one for professional content creation completely impossible.
Yes, a sysadmin could do a little coding, and remote into some systems on a smartphone, and throw an image onto a larger screen, and connect a keyboard - we've all experienced it. Yay! That functionality has existed in some fashion since the Palm Pilot was hip though, keep in mind. I did nearly all those things with my first Blackberry just for kicks. Telling me that a sysadmin can keep some servers running from a handheld is absolutely nothing new, and has absolutely nothing to do with "the death of the PC".