What product would that be?
Lacking a zombie upon which to do analysis, I couldn't tell you. The point is that there's plausible explanation for the observed state. Call it a "no-prize" explanation, if you will.
The point is that zomibes do not reproduce
Except for all those people who get bit, and turn into new zombies. Or, in the most popular version of the moment (The Walking Dead), the zombies who arise from folks who die of more natural causes.
In general, so long as the humans keep reproducing, and there's some available mode of new zombie creation, there'll always be some threat.
World War Z, for example, raises the point that zombies don't need to breathe air. So, those who fall or walk into the ocean can walk out some time later, on the same or another body of land, and cause further outbreaks.