Keep in mind that a rendering engine is far removed from a game engine. Typically, the game engine will contain a rendering engine, but that should only be a small part of the entire game engine. Beyond that, a software rendering engine really isn't necessary for actual rendering, it's great to know how things work under the hood, but learning D3D/OGL/Vulkan/etc is going to be more applicable to modern game development than a software engine. A software rendering engine is great for doing CPU occlusion culling though, but there you're working entirely with depth and visibility and pure optimizations to improve performance.