I wonder if it'll make it more stable because the passive investors aren't as inclined to randomly shift money around or pull it out. Even if they are inclined to pull it out, does the behaviour of the money being pulled out look much different when looking at the market in aggregate? It stands to reason even active investors would be heavily invested in those exact same companies and would be deliberately pulling money out as well? It seems to me that all the investors investing in the average just means the market will continue to be average.
So, yes, passive investing helps stabilize the entire market to trade in tandem, but too much is a danger to the health & stability of our market and economy as it enables misallocation of capital.
Is it your assertion that without index funds this would be occurring? We just saw deliberate decisions to buy companies like TSLA and GME that stronly argue against that.