Agreed, which is increasingly another reason I go for server/workstation class components. There are other professionals who need this class of machines, the CAD/CAM, scientific workstation, etc. crowd who e.g. Sun used to cater to. Small numbers for sure.
The other segment I can think of are the really hard core PC gamers, who I don't have any sort of reading on (decided to give that up in the mid-late '80s to limit damage to my hands on up). 5% might be too low, but I'd accept it as a working estimate. E.g. my parents would today be well served by small unconfigurable boxes as long as they can speak to a regular big monitor, keyboard and mouse, and I plan to move them from Windows XP to something non-Windows like Chrome/Chromium OS in a year or three. Hmmm, I suppose there's also the big machine to satisfy a big ego market as well. Big machine as in way overpowered to run MS Office for a Pointy Haired Manager.
The rest of the "PC" market is as you note, although it's not inconsequential. But it has very low margin, is trailing edge in every way, and by now has got to be "mature", in that replacements almost certainly dwarf new installations, especially with the declines in new business formation. I'm not focused on it, don't know how it will move, but again something like the machines that could well serve my parents, e.g. based largely on tablet technology cores, could well disrupt it and take it over from Dell etc. as they are today.
Which I mention because there's got to be a reason Microsoft is investing in Dell going private, although it might not make much sense (e.g. the acquisition of Skype which I gather put the final nails in the coffin of Microsoft's mobile phone ambitions, since that make them radioactive to the carriers).