Reproduction of virus.
“A computer virus, like a biological virus, must have a reproduction rate that exceeds its death (eradication) rate in order to spread. Each of the above obstacles significantly reduces the reproduction rate of the Linux virus. If the reproduction rate falls below the threshold necessary to replace the existing population, the virus is doomed from the beginning — even before news reports start to raise the awareness level of potential victims.” (source: https://help.ubuntu.com/community/Antivirus)
I can say same about enemies. If we stop making new, old will eventually tire out. New enemies are more fierce-full than older ones.