I wonder what a solution could look like. Perhaps keep the market cap weighting, but cap the weighting at a max $500b (or some sliding scale to prevent the top X stocks from composing more than Y% of the portfolio)
That would certainly be a way to control escalating concentration but at the expense of keeping index fund costs low. The Vanguard Total Stock Index (VTI) has an expense ratio of 0.03 - almost zero. Low expenses is a critical factor behind why index funds outperform active investing. So, yes, your proposal would work, but the expense ratio would up to implement the cap.
