Cubics vs. Quadratics is not as big an issue as you thikg. Cubics flatten down to quads just fine. (side not: Flash was all quadratics). The hard parts is that vector graphics just doesn't map well to gpus. Way too much CPU work involved. Another really big problem with vectors is that they are relatively unbounded. With raster you scale pretty linear in the number of pixels. Vectors jump from very easy to very hard all the time. And in graphics you need to care about the worst case.
Cubics cannot translate perfectly into quadratics, no. You can estimate a set of quadratics to build what appears to be a cubic, but they're not identical at any arbitrary resolution.
Quadratic bezier curves, unlike cubic ones, do actually map really well onto GPUs (given their basis on triangles) quite unlike what you've said here.
Sorry for not being clear. Of course you are right, they are not the same. What I tried to say was that in the context of rendering approximating cubics by quadratics will not be the bottleneck.
