The largest problem with buses in the region is that there are very limited options for some major travel routes. Very few buses travel between VA and DC, for instance, particularly outside of peak commuting hours. Within DC, there large sections of the city without reliable east-west routes - if you live anywhere significantly north of H St NE, there's really no frequent service that will take you into downtown or across the city without needing to transfer at least once. Once one has to switch bus routes in addition to taking the bus over rail, the time to make a trip can balloon immensely.
Even if Metro were to provide a bus alternative that parallels the rail routes, sending buses between Metro stations is typically very slow and inefficient, as there aren't typically fast road routes between them.
If the region invested in signal priority, (enforced) dedicated lanes in particular corridors and made some improvements to the bus network, buses could provide a viable alternative to Metro during line closures. Unfortunately it does not appear that any of that will be occurring anytime soon.