I've seen well backed arguments that the lowest lifecycle cost (maintenance) and lowest environmental impact surface (erosion, widening around low spots) for a well used trail/path/road is a raised, crowned & drained hard surface.
Has the NPS ever floated hard surfacing the C&O? Something like a polymer soil stabilized surface could be sensitive to the history/environment/aesthetic of the path.
I know, I know, all it takes is money.