What if, as in the song, humanity discovered they had five years left. No doubt.
Reaction? I think mass chaos cause enough people would just up and not go to work that the whole system breaks down. Too much crime for whats left of police to handle. Not enough dedicated to the food industry.
On a different note…what a great series that would be! With one minor change. The protagonist knows but humanity doesn’t. Run the show in real time. Five years. Of course eventually word gets out and then we go from there.