That's an interesting point. While it's definitely true that any major unwind of that size will disrupt the market, there's some reasons to believe an ecosystem dominated by active investors would do better.
First as mentioned before, active investors have discretion. There's strong reason to believe that as the sell-off's happening that they'd move into the most dislocated stocks. That acts as kind of a negative feedback loop. It wouldn't stop a market-wide selloff, but would keep things more balanced between single-name stocks vis-a-vis the rigid rules governing passive index managers.
Second, by definition active funds are more differentiated from one another. Passive indexing produces a mono-culture with analogous ecological risks to what we see in nature. The typical active fund only holds about 50 positions at any given time. So, on the whole while active investors in aggregate hold 0.3% of their portfolios in Chubb, at an individual level most funds hold zero. And some minority may hold 1%, 5%, 10% or more of their assets in Chubb. So if one fund fails, that's less likely to spread contagion to every other fund in the universe.
In that type of unwind scenario, some funds will do pretty decently, and some funds will do horribly. But the point is there will be a dispersion of results. That makes the market as a whole more robust. Panicking investors are more likely to re-allocate their capital from the bad funds to the good funds, rather than pull all their out in a flight to quality.
