DirectX is definitely not dead and is still more appropriate than WPF for advanced developers writing hard-core "twitch games" or applications with complex 3D models where you need maximum performance. That said, it's easy to write a naive DirectX application that performs far worse than a similar WPF application.
DirectX is a low-level interface to the graphics hardware that exposes all of the quirks of
whatever GPU a particular computer has. DirectX can be thought of as assembly language in
the world of graphics: You can do anything the GPU supports.
WPF provides a high-level abstraction that takes a description of your scene and
figures out the best way to render it, given the hardware resources available. Internally, this
might involve using Shader Model 3.0, or the fixed-function pipeline, or software. (Don't worry if you're not familiar with these terms, but take it as a sign that you should be using WPF!).
The downside of choosing DirectX over WPF is a potentially astronomical increase in development cost. A large part of this cost is the requirement to test your application on each
driver/GPU combination you intend to support. One of the major benefits of building on top of WPF is that Microsoft has already done this testing for you! You can instead focus your
testing on low-end hardware for measuring performance. The fact that WPF applications can
even leverage the client GPU over Remote Desktop or in a partial-trust environment is also a