The natural dynamic of an expansion is for above-trend growth to continue until demand overtakes capacity, despite the best efforts of policy to avoid cyclical excesses. The end of the story is particularly important.
Expansions do not die of old age or lethargy, a spontaneous weakening of aggregate demand, but rather of an accumulation of imbalances, specifically with demand outstripping the limits of sustainable level of input utilization and growth of output. The resulting rise in inflation becomes a threat to the continued expansion. Preventive medicine is therefore the best course of treatment.