You left out adding more people into the system, although that's usually considered in the context of demography rather than mortality. One accounting reason to consider it from this perspective, though, is that adult immigrants have typically been raised and educated at some other country's expense so their productive entry to the US workforce is free of any debt overhang, ie no public monies were spent by the US on educating so they're basically coming to help out with the national debt for free. Of course the flip side of this is a brain drain for countries that see too many people leave.