In 1999, at the dot-com peak, the ten largest S&P 500 stocks were about 25% of the index — and that felt alarming. Today it's 34.8%, and the top ten are almost entirely tech. The geographic and sectoral diversification that used to make the index defensible has quietly dissolved.
The 60/40 portfolio — 60% stocks, 40% bonds — worked for forty years because bonds went up when stocks went down. That correlation buffer is not working now. The 30-year yield is above 5%. SOFR futures have inverted. The 1994 analog is instructive: Greenspan raised rates unexpectedly, the bond market broke first, equities held briefly, then followed. The current setup is not identical. The part that matters is the same: investors rewarded for ignoring rate risk have accumulated a great deal of it.
Concentration is not a thesis. It is a condition. Conditions resolve.