If human civilization has been growing exponentially for a while, how have we not already hit these enormous galaxy-straining limits? The total number of humans still easily fits on just the Earth alone.
We haven't been growing exponentially for very long. Before agriculture was invented there wasn't much growth at all. We were limited by how much hunter-gatherers could hunt and gather. Growth happened when people expanded to some new territory, and that was limited by poor transportation options.
When agriculture was invented around 12000 years ago growth picked up, with the population doubling around every 1000 years. That lasted until around 5000 years ago, when the population was around 100 million. There wasn't much grown for the next couple thousand years.
Around 3000 years ago growth picked up and the population tripled by 2000 years ago. It slowed again until around the industrial revolution and then picked up quite a bit. The world population growth rate went from around 0.1% per year before the industrial revolution to around 0.6%.
The 20th century brought many developments that allowed for even more growth. That allowed the growth rate to hit around 2.24% in 1964. It's been declining since then, dropping to a little below 1% a couple years ago.
It's been pretty much the same pattern since we became Homo sapiens. When there is an increase in available resources or we get new technology that lets us better utilize resources we can have a relatively short period of exponential growth until we reach the limits of those resources and then growth slows or stops.
With the industrial revolution and then the developments in the 20th century we were able to have about a century of sustained high growth, but we're reaching the end of that expansion and the growth rate is declining.
We'll almost surely have more breakthroughs that allow for growth increases, but some people want to plan based on the assumption that we'll have such breakthroughs often enough that we can count on high growth indefinitely.
The point of the galaxy-straining limit calculation is that the past pattern of periods of exponential growth being followed by declining growth is something inherent in exponential growth. It's not something that can be escaped by finding new places to settle or more efficient ways to use existing resources. Thus, we should be planning on the recent period of exponential growth ending rather than becoming depending on exponential growth continuing indefinitely.
