I don't think that's a particularly accurate assessment of the idea behind index funds.

The point of index funds isn't "these outperform all pickers, so they'll outperform all pickers in the future."

I think the idea is more around a combination of:

- you'll have much lower risk trying not to pick the right picker (or pick the investments yourself)

- the median picker is probably not very good (approached in two directions: sizable pickers that hit on an edge will likely be copied until the edge is gone, and smaller pickers are extremely unlikely to have enough specialized info or skills to excel).