Isn't this already happening to some degree? E.g. UE's Nanite uses a software rasterizer for small triangles, albeit running on the GPU via a compute shader.
Things are kind of heading in two opposite directions at the moment. Early GPU rasterization was all done in fixed-function hardware, but then we got programmable shading, and then we started using compute shaders to feed the HW rasterizer, and then we started replacing the HW rasterizer itself with more compute (as in Nanite). The flexibility of doing whatever you want in software has gradually displaced the inflexible hardware units.
Meanwhile GPU raytracing was a purely software affair until quite recently when fixed-function raytracing hardware arrived. It's fast but also opaque and inflexible, only exposed through high-level driver interfaces which hide most of the details, so you have to let Jensen take the wheel. There's nothing stopping someone from going back to software RT of course but the performance of hardware RT is hard to pass up for now, so that's mostly the way things are going even if it does have annoying limitations.
And hardware raytracing is on the same trajectory as hardware rasterization: devs finding ways to repurpose it, leading to pressure for more general APIs, which enable further repurposing, until hardware raytracing evolves into a flexible hardware accelerated facility for indexing, reordering, etc.
Nanite is just working around an inefficiency that occurs on small triangles that require screen space derivatives, which the hardware approxinates using finite differences between neighbors, e.g. for the texture footprint estimation in mipmapping. The rasterizer invokes additional shader instances around triangle borders to get the source values for these operations. That gets excessive when triangles are tiny. This is an edge case, but it becomes important when there is lots of tiny geometric details on screen.
Why do you say 'albeit'? I think it's established that 'software rendering' can mean running on the GPU. That's what Octane is doing with CUDA in the comment you are replying to. But good callout on Nanite.
