Their longboats made them amphibious. Their ships were durable enough for the open ocean, but still able to land on any beach, or sail fair upriver.

For example, the Roman Republic had adapted to constant warfare and the steady stream of casualties caused by it. And then the Late Republic got into trouble, because Rome had become too good at war, and the required casualties didn't happen.

They had too many births before the potato famine -- it's the reason why it was such a disaster in Ireland compared to the rest of Europe -- and they continued up until a few decades ago. This happened during a time when it was unusually easy to move to "empty" land and colonize it.