M class stars have a life span of over 100 billion years. Since most stars in our galaxy are M class, it stands to reason that other galaxies have high populations of M class stars. While its true the more massive stars in the Hubble deep field are now burned out, they would have been replaced by new stars, because new more massive stars are always forming (at least for now). Indeed, if new massive stars weren't continually forming, there would be no stars in our galaxy as massive as the sun, because they would have all burned out since the Milky Way formed.