On the contrary, this just affirms their value-add.
As a decision maker, the reason you would outsource decision-making to 23-year old PowerPoint kids is so you aren’t responsible for the consequences of your actions.
“Darn, that strategy ended up killing people?? Too bad evil McKinsey told me to do that! I’m just a lowly corporate officer. I wouldn’t have killed all those people if they didn’t show us those PowerPoint slides! I got duped like the rest of you!”
[goes home and counts money]