And if 3^^^3 people are possible, 3^^^^3 people are probably possible too,

No, that doesn't follow at all. It's ridiculous to even compare the two numbers that way. I would agree that 3^^^4 people might seem somewhat plausible in that case, and 3^^^4 is already larger than 3^^^3 by a factor incomprehensibly greater than 3^^^3. Even 3^^^4 is probably already far excessive for what you need for your argument.