The concentration isn't the 'fault' of indexing per se. There are two styles of indexes used by funds/etfs.

1. Most indexes are market capitalization weighted indexes... which can lead to the high concentrations we currently see.

2. There are also equal weighted indexes. These are less popular for a multitude of reasons, not the least of which is the expense associated with keeping the fund equal weighted (the fund has to periodically - eg quarterly - buy/sell stocks to bring everything back to 'equal'

I am currently in the process of moving a portion of my allocation into an equal weight sp500 fund precisely because I want to lower my exposure to the largest ten stocks in the sp500.

Another way to accomplish that would be to buy a market capitalization weighted index consisting of mid size or small cap stocks - thus avoiding the concentration of the top 10. But that changes the overall portfolio in other ways (small cap factor). I decided to use equal weighting a portion of my large cap holdings because I feel it is a more precise way to address the very specific problem I am addressing without adding other variables.