David KostinDavid Kostin; Chelsea Jia Feng/BIDavid Kostin is seeing similarities to the dot-com bubble as "superstar" firms boost the market.Record-high market concentration will lead to a decade of weak returns, the strategist said.Kostin expects the S&P 500 equal-weighted index and mid-cap stocks to gain from a broadening market.David Kostin is no stranger to the ups and downs of the stock market.The Goldman Sachs chief US equity strategist has seen multiple economic boom-bust cycles during his more than 35 years on Wall Street.