Why no FTL and Infinite Worlds together? An explanation

[TGLS]

Originally Posted by Lovewyrm

And there, I simply have a hard time fathoming that all humans on all earths only uplift other earths, and that that is the true meaning of FTL and everyone else who secures more territory in a single, concrete reality is somehow wrong for doing that.

I dunno, I think you're thinking about it the wrong way. The world as presented the short story basically has two sets of dimensions: one where one travels to a differing spatial location, and one where one travels to a differing temporal location. If you travel to a planet in a differing spatial location, you have to deal with the possibility of radically new life/cultures/climate conditions/etc. If you travel to a planet in a differing temporal location, it's possible to see different new life/cultures/climate conditions/etc. but the deviation is lower. Sure, the planet could be utterly alien from your own, but it's less likely than when you travel to another spatial solar system.

The humans in the story looked around the nearby star systems, and decided that most of them were fairly hostile to human life (because it adopted Deathworld's "Earth is a hellscape to most other planets, but that make it hard for humans to thrive elsewhere"), so they made the economic decision to expand in the other set of dimensions. And it's not as if this is a one way move or anything, they have to build ships called "ARCs" to travel between dimensions. If Earth-66 gets attacked by aggressive aliens, then they can just hop over to deal with that problem, same as if Deneb IV got attacked by aggressive aliens.