Not the op but I wish downvoters would comment giving their reasons. I remember seeing something about this in the atlantic a while back, and it would be good to know if/why it's a bad argument.
Probably an interesting number itself knowing what the fractional amount of money flowing through Vanguard is as a proportion of the total.
Personally, I think that as more people put their money into index funds, active funds may look more attractive as they can potentially exploit market misprices quicker than passive funds can, but that's just me.
Some people upvote or downvote because they agree or disagree. They view it as a "vote" for the opinion.
Others upvote or downvote because they thing the argument is strong or weak, or maybe because someone said something nice and positive to another person or someone said something mean or hostile to another person.
I strongly favor the second approach.
One of the early slurs was that index funds were communism (pretty daft, but probably effective in the US). The latest is this scaremongering "what if all investment were into index funds" (answered several times in this thread).
From the perspective of index fund advocates, unsophisticated investors are systematically getting ripped off and the daft propaganda only adds to the offense.
This is probably why you're picking up an aggressive preachy vibe from advocates.
The empirical data is however unambiguous and there is now a lot of it.