The problem with this theory is that the current top-weighting is not all that much higher than during previous historical periods. There's a decent overview [0] covering the historical S&P 500's highest concentrations of the top 10 stocks which ranged from a low of 17.5% in 2014 to a high of 25.5% in 1980. It's a little under 30% today. Needless to say, the returns over the decades following 1980 were some of the most consistently positive in history [1].

[0] https://www.yahoo.com/now/top-heavy-stock-indexes-nothing-09...

[1] https://www.macrotrends.net/2526/sp-500-historical-annual-re...