I think it depends on what you define as "Post PC" , whether we really talking about "post wintel" or "post keyboard and mouse".
I think the precision and speed advantage you get from a proper keyboard and mouse setup vs a touchscreen, especially when you consider how cheap both of these items are will mean that they are around for some time even if it as an optional add-on rather than a core part of the whole interface.
I see the "post pc" as more of a rethink of the PC minus the general historical baggage that various versions of Windows etc have carried with them for last 20 or so years.
What I think people are really clambering for is a better user experience from their software.
Let's take the filesystem as an example.
Years ago it made sense to have an OS with the concept of files and folders being exposed to the user.
Why? Because you needed to be able to group your stuff together, either things for a particular project or because they were of a particular file format (word documents , spreadsheets etc).
You would also have "drive letters" which told you whether it was on the "C drive" (inside the computer) or the "A drive" (portable). This is no longer important since we can trivially sync most things to the internet regardless of where the bits are physically stored. In fact Unix doesn't even have the concept of drive letters at all.
File formats can now easily be replaced by Mime types and meta info since fast SSDs and indexes can be used to search either the device itself or cloud storage very quickly. This means it is easy to have an app that loads and immediately provides a list of all the documents which it can work with.
The only requirement that is left is the "grouping", folders are pretty bad for this because you can only group in one dimension. A better system might be some form of "tagging" for example tagging as "work" or "my project" etc.
By removing the filesystem from the user's consciousness you have already massively improved usability and I think this is probably the biggest thing that has made the iPad so easy to use because it essentially turns the install process for an application into a 1 stage thing. Combine this with having updates automatically install silently in the background and an application ecosystem built around the idea of the internet and web as a first class citizen in the OS rather than an extension of it and you have a very seamless experience.
The physical "input into the device" landscape though I see as something blown open and I really don't think there needs to be one dominant paradigm here.