Regardless of the topology one chooses, the basic problem of building an amp that measures well and sounds good has been solved. Class D isn't going to bring about entirely new levels of fidelity. We already have amps that will drive any load with vanishing levels of noise and distortion. And a lot of people don't much like them. I think Nelson's point is that the challenge of building a well performing amp has been mastered and pretty much everything you see today is just someone's opinion of how to best implement a hand full of basic topologies in a manner that suits someone's tastes. In that, amp design is art. That doesn't take away from the technical challenge.