Yelich might have been a 1ish WAR improvement...but I think he would've been a 3-5 real life win improvement.
This offense is dysfunctional, and it has been for almost 2 full seasons now. I'm convinced that one very good player (like Yelich) contributes much more than a pretty good platoon like Almora/Happ, and it's something Fangraphs hasn't really figured out how to quantify yet. We'll see as metrics continue to evolve--in 5 years, I think we're going to look back at the 2017-18 Cubs as teams who placed too much emphasis on overall run scoring over being good on a day-to-day basis.