- There is more/richer information available on people who keep journals, so it's easier to make the case for their greatness.
- If someone does great things but no record is kept, they are not acknowledged.
I enjoyed the stories of his grandfather, but the flawed reasoning in the argument annoyed me.
It does not address the (likely many many) people who keep journals but will not be considered great.
It is a valid point that keeping a journal does increase your legacy as it becomes easier to write memoirs for your grandchildren. Even if no one else will read it, they will have a vivid portrait.
I don't think it means much either way as far as whether I'm "great", whatever that means anyway.
I think keeping a journal has been vaguely useful.
That might be a better argument for it.