There is some point to that. We are perfectly adapted to our planet because it formed us. Colonists on another planet may not find the same perfect conditions a

There is some point to that. We are perfectly adapted to our planet because it formed us. Colonists on another planet may not find the same perfect conditions and it's likely that even to get a partial match (say Mars like conditions) they'd have to be many, many light years away from each other, with little outside support. And they'd need to adapt to colony ship life first, then to life on another planet. They'd more or less evolve completely independently and it's conceivable that having these limitations will mean they never reach the same potential as the original humans did on Earth. Usually the comparison with colonists of the past pops up. But they had the distinct advantage of being perfectly adapted to the planet and many civilizations still perished. Seeding another planet with humans raises a lot the bar for successfully developing a thriving civilization capable of reaching the levels required to further spawn successful colonies. And a philosophical thought: would they even still be humans after (tens of) millennia away from the home of humanity, in vastly different conditions from the ones that evolved us? Would they be "our" thousands of colonies?