The Fed is growing increasingly concerned that this expansion will end like the last two, with a collapse in asset prices that brings down the economy. That concern will lead the central bank down the path of excessive tightening. Worse, that logic misses a key point. In both of the last two cycles, there was a sizable imbalance in the economy that extended beyond financial assets themselves. So far, the current environment lacks such an imbalance. That suggests the expansion ends with more of a fizzle than a bang.