The argument isn't "economic growth has to be growth in number of atoms". It's "if the economy grows at say 2% for say 10k years, without anything happening th

The argument isn't "economic growth has to be growth in number of atoms". It's "if the economy grows at say 2% for say 10k years, without anything happening that today's physics says is downright impossible, then at the end of that time humanity will (1) still be physically confined to this galaxy and (2) be so wealthy that we will have as much wealth as the whole present-day economy times the number of atoms at our disposal; hence either (a) that growth will slow dramatically, or (b) something will happen that today's physics says is downright impossible, or (c) some other change will happen that decouples us from our component atoms so radically that it's possible to have a whole present-day economy's worth of wealth per atom or more; none of these options can reasonably be described as "business as usual".