> The hard part is hardware acceleration for the GPU

Is it, though?

How Hard Can It Possibly Be to just do a software GL renderer that emulates a mid-2000s Radeon, these days?

At what resolution. You're not going to software render 4K120FPS even with 2000s graphics. But you also don't need a software implementation since translating to a host API isn't really any harder than that (and often much easier). And this already exists in Wine.