and we never get anything better than fusion drives (and Bussard ramjets don't work), then maybe a high density of these "rogue" worlds will allow the (very slow) colonization of the galaxy.
If there are roughly 1000x as many these large planetary bodies floating in interstellar space as there are stars, then perhaps it'll be feasible to travel to them in tens of years traveling at speeds achievable by nuclear fusion (hundredths of "C"). Then, using the resources there, colonies could be set up. Eventually, these will sprout new colonies, further pushing the boundaries of inhabited space until finally they reach a star.
This scheme of colonization would be unlike anything the western world, even in the days of years long voyages via sailing ships, has known. Perhaps the closest would be the voyages of the far flung polynesians who managed to spread across the vastness of the pacific ocean over a period measured in centuries(?). If any of them made it to South America (some say they did), it would be like these future voyagers making it to the next star.
Of course, we all hope for a Star Trek/Star Wars future with warp/hyperdrive bringing the stars within an afternoon's jaunt. Failing that I guess the runner up desirable future would be the hyper broadband interstellar communications network in which our downloaded selves could be digitally transferred at the speed of light to the next instancing hub (such as in Greg Egan stories of the post-singularity future).
However if neither of those pan out and if we don't learn how to make/harness anti-matter, micro-blackholes, zero-point energy, giant laser driven solar sails or ??? then perhaps this is our most optimistic future.
Maybe with immortality and suspended animation it won't be too bad. Slow trips around the galaxy indeed