the machine god is actually the void dragon one of the last four surviving four C'tan in the eldar mythology, and there are hundreds of thousands of necrons in comparison to shear numbers of humanity there hardly a threat until the C'tan wake, Also the eldar using the infinity circuit believe that the only hope for the survival of the galaxy depends on humanity..... any who back on track.
Chaos space marines are not immortal there shear age is because of the time warping effect of the Eye of Terror and on a one on one basis are superior to a average marine because the first founding marines were better in every possible aspect also the numbers of Chaos marines are not finite because most of the legions recruit from worlds within the Eye or Maelstrom or are traitors from the marine chapters. for those able to read through my tangent i salute you...........
p.s. the C'tan will eventually destroy the eldar, orks and other surviving warrior races created be the old ones, crush humanity proceed to enslave them, finish their pylon system (some of which are on cadia) to seal of the warp from real space for ever, then wage wars against each other until the night bringer and the void dragon are left at which point the Outsider will swan in with the Tyranids and remove all life.. thats it every body dead....
the whole 'nid thing with the outsider is my own theory please feel free to ignore it