This fallacy is used to trip up pro-lifers, particularly in the context of social security and immigration, where the demographic collapse of the West plays a key role. Evagrius' is a response to the statement that the impending collapse of social security was due at least in part to the fact that the generation which initiated it also murdered a massive proportion of the generation they expected to support it. Additionally, the influx of immigrants from poorer countries in the Middle East, North Africa, and Latin America is directly related, and possibly caused by, the fact that millions of would-be workers have been aborted or contracepted away.
The proper response is that the aborted generation wouldn't be on welfare, they'd be doing the jobs "no American would do." Well, no American can do any job if he's dead. Except run for office. Or vote from time to time as the need arises.
The great influx of women into the workforce towards the end of the first half of the 20th century also plays a part here, both in taking the jobs that the murdered infants would have taken, and in depressing wages. Between the immigrants and career women, you may have enough to account for not only all of the murdered unborn children, but a good chunk of those "contracepted" away as well.
JMM