This. Just, yeah. He was already "hope" for some of us, and that hope is now all but entirely gone. Or, wait, no. We, the WoLs are hope, and he's just a vessel for the royal Allagan blood. He's found his own way to help out! *sob*
Remember Nero's tomestone. There's much more to it than just a couple of notes on Allagan royalty, so I wonder if the data on it can be of more use to Cid now that he's been inside the tower itself.
Get it? It's a Hope joke! In rather poor taste, though... (We also saw Hope again five minutes later in XIII-2. Trying to think of when a scene like this was supposed to be a final goodbye in other media, and it's not coming to me.)
The practical problems with using the tower all boil down to "basic human nature." Sure, we can create a world where that basic nature will no longer endanger the world, but should we? Even Hayao Miyazaki, of all people, said no. It gives, and people are going to to take. If it can provide everything people "hope" for, people won't need to look elsewhere, and that's already been portrayed as having serious consequences. G'raha... probably won't be treated like a hero once his "purpose" is served, because that's just how people are.
He didn't forget... in the Japanese script. This has more to do with "exchange of information" streamlining, EngLoc's worst habit, so it's more that he just isn't repeating information the player already knows. This is also the basis for my "one last shot" theory, since using it for a single, massive application and then letting it shut down for good is a fairly poetic way of letting it symbolize hope without any of the explained drawbacks. We might even get a scene were G'raha tries to seal himself inside again, before his control wears off, but Cid or Ramnbroes stops him by waxing poetic.
I get the feeling we were supposed to be comfortable with letting it end like this, because "yay hope," but not because they had no intention of planning more. There's too much doesn't add up as neatly (in a narrative sense) as a proper wrap-up would.