I'd say they're building toward him starting right now.
Also, he's been a savior in a few games out of the pen. It's not like he's been wasted in meaningless outings. Every one of them has been important.
What you said was
Also, you can add pieces during the season- like Eovaldi and Pearce in 2018, to put you over the top, so no, you don't have to be one of the best to start the season.
The last part makes no sense in the context of your argument.
The concept behind WAR is quite logical. Positive plays and negative plays are translated into runs. Every 10 runs is estimated to be worth 1 win.
It's just an estimate of a player's total contributions.