Good point.They want Reigns to spear and Superman Punch Fiend 300 times and win the title. They are also very well aware that Reigns will be booed out of the building when he wins; not good for optics. On the other hand, they know that Drew is over, and if they book him to win, the crowd reactions will be insane.
If they book Drew to lose, then Fiend vs Reigns will main event. If they book Drew to win, then obviously Lesnar vs McIntyre will main event.
The positioning of both matches on the card will give away the result.