The point is that research on sugarcane has little to no bearing on what research on stevia would uncover. The fact we think sugarcane is safe is not evidence in favor of stevia. Just because both happened to evolve with little human interference doesn't correlate their effects on the body in any way.

The exception is if we can prove the two operate under the same fundamental mechanisms within the body. But there's nothing different about natural growing plants and man made substances under this sort of research. A molecule is a molecule.