but if you consider how damn many human's there are... Think about it only pure human colonies were hit, to cut down on witnesses, and at that only about half of the "outer colonies" were hit... most of those were still being colonized, earth has 11.4 billion, and 250,000 in space, let's take an established colony Eden Prime, between 3.7 to 3.9 million, Horizon, which like many colonies were still being developed had 600,000+, so it takes a few million to build a reaper, think of what earth could provide... that aside
with DLCs and appropriate Army building over the first two games I believe that humans can win