It's all cool and awesome, but why put all those resources into new renderer, when one could contribute to Blender and such, which are way more mature?
Appleseed has also been under active development for 7+ years; so not that young, especially considering that physically-based renders really only started coming into full production use 3-4 years ago. There are still many visual effects studios using approximation-based setups today, so I'd say there's lots of room for "new renderers" to carve their niche.
I know Blender is not a renderer, but they include 3 renderers in the package: Bledner Internal, Cycles and, Blender Game (This one is Real-Time though).
1. Because it's fun.
2. Because you're using a different program than blender.
3. Because you want to work on code that you know wouldn't get accepted in the blender code base anyway.
4. Because you can.
I have contributed patches to Blender that got accepted. Even I feel the urge to write my own render engine, to try out different approaches and just so I can claim "I wrote that".
Blender is not a renderer, it's a modelling and animation program with a built-in renderer, Cycles. But it can connect to other renderers, too.
Cycles is not that old. It replaced Blender's prior renderer just a few years ago.