I meant https://en.wikipedia.org/wiki/Colonization_of_trans-Neptunia... where they're colonizing those spaces as a place to live -- supposing civilization has already spread through the solar system, we're not talking about needing some kind of super Manhattan project to spread a little farther out. You just need people who feel crowded in the already-full habitats.

Of course this is speculation, but it's one chink in the case for the cost of travel stopping life from spreading. Personally I expect fancier higher-tech faster means in the actual future, like Forward's Star Wisp scheme.