You'd have to find a ROW and build new tracks to do commuter rail, and connecting to Union Station would be a major (read: expensive) problem. I agree that Metro out that far is not an ideal solution, but most of the cost savings traditionally associated with commuter trains don't apply in this corridor. A commuter line out that far would be nearly, if not as expensive as a Metro line... and considering the Metro line is actually in planning and well-funded past Tysons Corner, it doesn't make much sense to fight reality.
As far as Richmond: You could be right about the in-town stations. I don't know that part of the city well at all. I don't know about a station in Doswell, though. Unless I'm missing something,
it's not exactly a large population center.