That's a good point. You could avoid that problem by giving grades relative to the best product on the market, e.g. anything that doesn't use more than 10% more than the most efficient model on the market gets an A.

Regrading every year seems to be about the same as what I propose. I guess the actual numbers used for the current rating are based on actual efficiencies of the models on the market when the regulation was introduced.

Only if consumers pay any attention to energy efficiency labels. I strongly suspect that with washing machines and dryers, they simply ignore them and look instead at the other features (particularly load size and cycle timing) and the price.