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I think the extrapolation is well justified because currently both indices are dominated by a few mega technology stocks, as illustrated by the chart below. Source: ETF.com Price and growth expectations QQQ currently trades ...
While I might agree with you about the top 5 stocks durability relative the old top stock's durability, that is cheating. Had you asked people back in 2000 about durability, people would have insisted ALL of the top 5 were going to last forever. I can't really see Google being dethroned...