One series (that I'll leave unnamed to not spoil anything) suggests that the limiting factor isn't habitable planets, but wars ended up being fought over limited availability of metal in the universe. You can build your own space ships and stations, but eventually the raw materials have to run out no matter what you're making.

There are of course other limits that may or may not be solvable, like most of that metal being stuck in relatively deep gravity wells. But it's still an interesting thought.