Nice presentation. Something I've seen a lot over the decades is internal politics. One huge advantage of not using a brainstorming team is 99.999999% of humanity is more creative by sole mass of brain power than the team will be (Six billion people vs a team of ten people) so almost all good ideas, for any problem, will be discovered outside the team.
If the team is in charge as a group of doing the brainstorming, 99.999999% of the good ideas will come from non-team individuals who will be automatically shot down as "not your job" "work on your own team" and similar primate dominance games. If you brainstorm individually, that 99.999999% of good ideas has a better change of getting implemented. Groups hate outsiders (and by extension, their ideas), without a boundary there is no group.
Another internal politics problem is reward. Teams exist to funnel all success upward and channel all failure downward. Everyone knows if they actually have a solution its wasted on the team unless they're the leader. There are financial and career pressures for teams to fail so individuals can keep the fruits of their labor, so to speak. This can lead to very low team performance.