I'm not assuming it's going to fail, I'm just saying that the exponential gains seen in early computing are going to be much harder to come by because we're not starting from the same grossly inefficient place.
As an FYI, most modern computers are modified Harvard architectures, not Von Neumann machines. There are other architectures being explored that are even more exotic, but I'm not aware of any that are massively better on the power side (vs simply being faster). The acceleration approaches that I'm aware of that are more (e.g. analog or optical accelerators) are also totally compatible with traditional Harvard/Von Neumann architectures.