Assuming we don't extend any of our current players, don't make any new signings or trades, and do nothing to improve the current state of the farm system between now and 2020ish, then yeah, the following years are going to be pretty bleak...but surely no one believes that's going to happen. That our farm is much weaker than it was a couple of years ago, and that almost our entire MLB team will be set to hit free agency within 2-3 years of each other, is disputed by no one, but the "cliff" theory seems to assume that the Red Sox will do nothing in the face of this and will have no choice but to rebuild, which I heartily disagree with.
As I've said before, if this is a bad team in 2021 or whatever, it will be because we didn't play our cards right between now and then (which could be said of any organization!), not because it was somehow predetermined by moves that have already been made.