The may not need to speculate about this, if a Spreading Disaster Map Graphic crops up in the opening credits.
Another common staple of the Zombie Apocalypse is that the zombies are often not the most dangerous enemy that a survivor will face.
If they ARE actually competent, they'll just also happen to be evil.
The collapse will also take place very quickly, over a period of weeks or months, instead of years.
Failure is often the only option in these stories; rarely do they have an ending that could be considered "happy" by typical standards, or indeed one where humanity survives as a species.