What projects like Slug and Vello rather show is that GPU coding remains so obtuse that you cannot tackle an isolated subproblem like 2D vector rendering, and instead have to make apple pie from scratch by first creating the universe. And then the resulting solution is itself a whole beast that cannot just be hooked up to other API(s) and languages than it was created for, unless that is specifically something you also architect for. As the first slide shows, v1 required modern GPUs, and the CPU side uses hand-optimized SIMD routines.
2D vector graphics is also just an awkward niche to optimize for today. GPUs are optimized for 3D, where z-buffers are used to draw things in an order-independent way. 2D graphics instead must be layered and clipped in the right order, which is much more difficult to 'embarrassingly' parallelize. Formats like SVG can have an endless number of points per path, e.g. a detailed polygon of the United States has to be processed as one shape, you can't blindly subdivide it. You also can't rely on vanilla anti-aliasing because complementary edges wouldn't be fully opaque.
Even if you do go all the way, you'll still have just a 2D rasterizer. Perhaps it can work under projective transform, that's usually pretty easy, but will it be significantly more powerful or extensible than something like Cairo is today? Or will it just do that exact same feature set in a technologically sexier way? e.g. Can it be adapted to rendering of 3D globes and maps, or would that break everything? And note that rasterizing fonts as just unhinted glyphs (i.e. paths) is rarely what people what.