No end to the road...
The PC is not the ultimate computer, it's just what we've got a lot of right now.
Just as the Model T Ford brought motoring within reach of most everyone, so has the PC done. Moore's law alone assures us that the desktop machines of the next decade or two will blow away what we've got now.
There will still be room for plenty of further development and innovation, and there are still valid alternative architectures such as Sun and SGI who make really good machines with more grubt than you can poke a stick at.
Technology is a journey, not a destination...