drmwvr said:
I believe that is exactly the plan, get rid of the alpha species and come in afterwards with little to no resistance. They can let the humans extinction play out over the next 10 or so years while they are traveling towards earth. At most, if no new babies are born, we (humans) will cease to exist in 110 or so years. This assumes babies are still living, which the show has not shown any, and in fact hinted in episode one (in the hospital) they don't.
One episode there was mention of a specific number of deaths and births that day, so I assume that the current pregnant women are still "finishing the job" and there are still healthy babies being born. For the following few months at least. Beyond that initial supply, there's no indication that they're making new ones.
Of course, there was also a mention of orgies in a different episode, so maybe thats actually a non-issue.