I mean, in some sense anti-trust laws do this. But you're right it would be impossible/dysfunctional at the level of retail investors. More plausible is some s

I mean, in some sense anti-trust laws do this. But you're right it would be impossible/dysfunctional at the level of retail investors. More plausible is some sort of rule preventing a single index fund from taking large positions in multiple firms the same industry; retail investors could then invest in multiple index funds, achieving the same diversification but with increased difficulty of collusion. Unclear whether that would be worth the higher costs, though, and inevitable ambiguity about what constitutes an industry.