If the O's were to ever leave Bmore (doubtful), the Nats would only gain marginally in attendance. Unlike the MANY DC area fans who flocked to the O's in the absence of baseball in DC, there is a visceral, longstanding hatred of all things Washington in Baltimore.
You could probably count the Bmore football fans who "adopted" the Redskins during their football "drought" on two hands. They became Steeler fans before Redskins fans which, given the current rivalry with Steeltown, tells you a lot.
In some neighborhoods where all things Baltimore was a big deal, sure for some, but hardly in the entire area. This is the market that wasn't even selling out World Series games (per sports writer Tom Friend, who had a great article or two on deflating the latter-day Baltimore fandom bluster), so it's the diehards you describe who were not in abundant supply. Plus, as the Skins had an actual waiting list and sellout streak, you were never going to get definable numbers of switcheroos in terms of ticket measurable like you could attempt to do with Birdland.
