I think that your point about places like Philly and Baltimore being easy to rehabilitate in 15-20 years makes sense. But there are still large areas along the D.C. orange and red lines that are virtually unwalkable without putting your life in danger. For example, I used to live what would have been a 15-20 minute walk to the Vienna station on the D.C. Orange line, but to get there, most of the trip was next to fast roads, with no sidewalks, and a trip over two overpasses. I made that trip twice before giving up (and after almost getting hit by a handful of cars both times).
There are parts on the blue line that are pseudo-walkable once you get off the system, but places like Franconia-Springfield are death traps.
I do agree though that even those areas are better than they were when those stations were built. However, outside of a very limited "walkable" area of a couple of blocks in the best cases (like Dunn Loring or the Two Falls Church stations), you'd be hard pressed to live anywhere but immediately next to the station and not drive.
I really think that it's because most of the D.C. area grew up post-automobile and is designed as a vast urban sprawl. Things that might have a positive influence, like new lines on the system, have taken 30 years to get started on (Silver Line).
I'm however, happy to see new developments along that line, where those lessons have been learned and they are basically rebuilding parts of the urban infrastructure to make it more downtown, like Tyson's Corner and Reston.