> Ultimately it's all about market returns. If other indexes add it and outperform then eventually money will shift to those funds that do better.

That's like saying that if Nvidia performs way better than an index fund, then the index fund will shift to consist only of Nvidia.

In any given year, there are plenty of index funds that outperform the S&P 500. They don't freak out over it.

S&P 500 is volatile over 5 years - I'd argue even over 10 years (see the charts at https://blog.nawaz.org/posts/2015/Dec/pay-down-mortgage-or-i...). The whole point of investing in it is for much longer windows.

So yeah, perhaps after 10 years they'll change once they'll see other index funds doing better, and have data to back up that in the long term, early inclusion didn't hurt.