They've been able to bank about 10 WAR per year from Harper and Murphy alone. Neither their system nor the 30-35 million they currently spend on those two are going to bridge that gap once they're both free agents, that's the point being made. The point isn't that the Nationals are going to be terrible, the point is they are almost certainly going to get worse by a fair amount unless they want to push the limits on luxury tax penalties. Given the Lerners are so thrifty to be as deferral-obsessed as they are, I can't imagine that's something they'd be interested in doing in general, and especially if anything is remotely imperfect(Bryce Walks, one of Scherzer/Strasburg gets hurt in '18, etc).