PCs last longer these days, are no longer subject to significant performance improvements every couple of years, aren't frequently lost or dropped/broken/stolen, and don't come on plans with carriers that have you in a constant upgrade cycle.
Despite what you've said, non-PC devices are for core tasks (email, web browsing, messaging, etc.,) a highly-refined, elegant and convenient experience for many end users. Given a person who uses their computer almost entirely for the tasks I've mentioned (and there are plenty of this type of user), would it not be right to recommend them an iPad or other tablet?
What, exactly, would that user be gaining from a full-fledged desktop or notebook PC? That question is getting increasingly difficult to answer.