The point isn't that physical limits will only be a problem in 8200 years, it is to counter the defective idea that such growth can be sustained indefinitely. W

The point isn't that physical limits will only be a problem in 8200 years, it is to counter the defective idea that such growth can be sustained indefinitely. We don't have access to all the atoms in the galaxy, and can't even make use of most of the ones we have here on earth. We will hit a wall much, much sooner.