It seems to be just the very basement level of an open source Rive-type project. You would need to build the runtime around this renderer and then of course the authoring app. It would still be quite a lift, but hey, success does start with high-performance vectors.

More likely, I see perhaps other projects with vector-rendering needs (in-game UIs, video creation tools, svg tools, etc) perhaps studying or adopting the render to make their own graphics more performant.

But to answer your question on "who pays for this?" I can raise my hand and say "I do!" and it's a great value. But just like Copilot probably isn't the best monthly investment if you're not doing quite a bit of coding, Rive probably isn't the best investment if you aren't doing quite a bit of animation.