I think you’ve got it backwards. The weaker your economy is compared to your opponents the more aggressive you have to be. Add to the fact that the sheep bonus doesnt scale at all, it needs to be used to fuel aggression sooner or later. Besides, a drush in of itself is as much of a defensive strategy as it is offensive, if not more.
Its not so much that other civs can’t match your army numbers or even surpass them, but at least then you defend from there knowing they’re not happily booming and running away with the game.
This follows the time tested RTS formula: When they attack you defend, when they defend you expand, when they expand you attack.
Interesting discussion.