Humans are the most adaptive creatures known. We've colonised all four corners of the world and live for periods of time outside in orbit above the planet. There's no reason that humans won't spread across the known galaxy if it becomes both technologically possible and economically viable.
Ah, but the point is that we're very narrowly adapted to a very rare set of environmental conditions and we've only been around for a very, very, very, very, very small period of time. We're talking a blink
If you just measure success by 'how long you stick around', the dinosaurs are still outplaying us tremendously.
I'll quote myself:
Something much more rapacious and much more adaptive would probably do a lot better (something that needn't haul around a bubble of its absurd niche environment.)
We're far from optimized for galactic domination. I'm sure there are designs out there that can do everything we can do, but do it better, and then do a lot more on top of it.
Saying that 'we're the most adaptive species known' is both silly (every other species we know is a product of the same environment) and dubious (cockroaches will probably survive a lot of things that we, as a species, couldn't.)
We can't live anywhere unless we duplicate optimal conditions. Don't believe me? Sleep in your garden naked and don't go back inside for a week.
Precisely! To get anything done, we have to drag a little chunk of our stupid home with us. Whereas Bob the Post-Singularity Xenomorph is a perfect, immortal machine, endlessly adaptable, at home in every environment from the cold of interstellar space to the accretion disk of a black hole. If it's got matter, and it's got energy (and arguably only one of the two is neessary) s/he/it can eat it and make more of itself!