I'm a little sad at this revelation, though if the old man has no idea about the bigger picture, why is there a Katastrophe counter on the Gantz? Is Katastrophe really the end of the world? I'm guessing the counter has finished now (surely its been over one week now in the timeline since Nishi showed it off) and the real Catastrophe is the old man's company cannot bet on the hunts now, dunno why to be fair, but it would be catastrophic to him at least, and we can see the Gantz ball probably was invented like 31 years ago like discussed before.
The Seijin, possibly created by the scholars also for the purposes of the hunts (which suggests they release the Seijin on purpose to be slaughtered...).
The real plot-hole is the vampire race that had been around for 200 years then attacked recently by a Gantz team (quite possibly in Izumi's first run). This seems to go with Nishi's statement on his web page of aliens arriving and then the teams sent out to get them (which would rely on a sensor of sorts in the Gantz ball in a certain area effect - a diagram of such a radius over Tokyo can be seen in the Gantz manual which supports this).
It then seems that a drip-fed real-alien invasion was known of by the old man and company and the vampire race are a group of humans that fell victim to an alien virus 200 years ago, from a UFO crash like in that once Dr Who episode. Thus the old man realised there would be a fair supply of Seijin "targets" arriving on Earth. The only thing is, what kind of jerk translates his daughters code of a way to defend the planet from invasion then thinks, screw it, lets make it a reality show we can bet on, thus saving my ailing company first and then the planet if these people dont screw up...