Comment by psvx

Even if you ignore volumetrics and splines, a production renderer still needs tons of features. Gradually adding those increases overall system complexity exponentially as of course new features should not break old ones, and need to be backward compatible (scene format wise) as well. Consider f.e. different sampling types for indoors/outdoors/material wise, handling millions of triangles with limited RAM, subdivision surfaces and sub-surface scattering, many many types of materials your users expect you to support (and precompiled combinations there of), speed expectations etc. That's really just scratching the surface about what's in a production renderer. As they say, the devil is in the details :). Of course if you render two spheres on a plane, yeah you can write a tracer for that in 10 minutes and even compete with others.