The standard rendering pipeline is still fairly fixed and mandates shaders for vertices and fragments. With software rasterization, there are no more vertex or fragment shader, only compute. And you don't use the hardware rasterization units - you rasterize the triangles yourself and write the results to the framebuffers with atomic-min/max operations. You decide for yourself how and when you compute the shading in your compute shader. This can be multiple times faster for small triangles, and 10-100 times faster for points. And once you do things that way, there isn't much point for graphics APIs anymore - everything is just buffers and functions that process them.

Why is there a pipeline at all? Give me a framebuffer, a place to upload some CUDA or equivalent, and some dumb data pipe between the host program and GPU program.