Yes, index funds follow the index. When an industry or sector is undervalued, it is because this sector is inactive and can't get the attention of the market, but there is relatively large room for making money, and the index with low valuation will also rise.

If the valuation is low or high, it will slowly return to a reasonable state, but often this time will be very long. If investors can accept that the index can go out of the trough, they can buy it. Relatively speaking, the subsequent profit space and probability are greater.