Ripleys escape

I remember reading somewhere that the only reason Ripley is able to defeat the Alien is that it was engineered to only have a limited life span.It does seem sluggish at the end of the film. This does make sense i suppose, the space jockies use these 'Bio-weapons' to do a job and then they expire,leaving the space jockies nothing to clean up i.e. dangerous Aliens running free. What are your thoughts on this concept?