Alternatively, you could notice that the abnormally-long lifespans match a pattern of exponential decay towards a new equilibrium. This would be consistent with a hypothesis that there was a lifespan-extending factor involved, which effects grew weaker from generation to generation. The factor could perhaps have been environmental, or genetic.

I don't subscribe to this view, because I no longer believe the stuff described in the Bible actually happened as described - but if I were trying to treat the Genesis stories as real, this would be the approach I'd take to reconcile them with scientific knowledge.