The acquisition of Babe Ruth is still probably the biggest moment in Yankees history. Maybe they still become the force of the 1940s and 50s anyway, but Ruth put the team on the map. Two decades after he left, they were the only team left in New York until the Mets came. Nowadays, they’re the most famous team in baseball. It’s fairly likely none of this happens if he doesn’t don a Yankees uniform.
Everyone knew it was a big deal when he was first sold to the team, although I doubt anyone knew how big at the time.