We tend to think of Servers as super machines. But in fact there are a number of different "Servers" types. Media, file and web are just several that come to mind. Most servers are specialized use machines.
Desktops are general purpose machines that have to do many things well. So many are over powered in memory, Video performance and storage capacity, for what they really do. I know mine is, at least until the next best game comes out that is!
I know that the RasPi as a web server is adequate for the average personal page. It would not work well as a retail site. As a Media server it is also adequate and as a file server it is adequate. But many people stuff all three into one RasPi and wonder why they need (want) more memory and more processor and more storage. A possibly better solution in these cases would be to run two to four RasPii, each with a specific use.
That type of solution will not help the RasPi as a desktop replacement. Any five year old used PC (probably any ten year old used PC too) can run circles around a Raspberry Pi as a "Desktop". I usually give my old system away to someone who needs it.
ARM boards are getting very powerful, and they have came a long way towards meeting a majority of peoples needs for Internet use, but they are not Desktop/Laptop powerful yet. The desktop is going to hang around, at least for a while yet. Mainly this is because of their power, versatility and their easy of use. For me it is screen size and its power for gaming. Just cruising forums and such is actually easier on an iPad or other tablet. The same for reading an e-book.
I think the next RasPi will be more powerful if for no other reason than ARM SoC's are continuing to advance. The foundation seems to think a replacement is around 2016 or so. That really is not that far away. They are going to have to start looking soon if they want to make it by the end of 2016. The process should be easier as they have a whole lot more experience under their collective belts now.
The next RasPi will still not qualify as a desktop replacement. It will stay in or under the current price range, except it might need to be adjusted due to inflation. More memory maybe, but not 2GB, 4GB ,or 8GB. A faster processor, probably as the average is about 1.2GHZ or so now. Multi-core, maybe as that is getting to be about average for ARM now. I do not think that the Foundation will be looking specifically for a multi-core, but might have to settle on one. USB 2.0 and Ethernet I think you can bank on those. And it will be as compatible with their current work as they can get it! I do not expect the features to change a whole lot! Maybe their experience, or the chip, will allow them to work a new feature or two into the new design.
I guess we will know for sure in 2016!
Just my opinion on the matter.
512MB version 2.0 as WordPress Server
Motorola Lapdock with Pi2B
Modded Rev 1.0 with pin headers at USB
(RS)Allied ships old stock to reward its Customers for long wait!