Losing their income has a greater economic impact than just their own wellbeing. Pensioners might be retired but they are still consumers with debts of their own. Homelessness alone is very costly to the U.S.
Pensioners are window-breakers, in today's society. They consume, but never produce. (This used to be less true, with grandparents providing childcare etc) This doesn't mean we should kill them all off! But the consumption of a pensioner (or retiree in general) is more akin to digging a ditch and filling it back in than it is to anything productive.
You can't build an economy on consumption alone.