SPY wasn’t just a hit product, it was an inspiration for an entire industry to take off. But it didn’t happen overnight. It took about 10 years for the ETF structure to be utilized for other asset classes and strategies, which today seem normal but at the time were revolutionary.On this...

SPDR S&P 500, or SPY, is the world’s largest ETF today with about $240 billion in assets, but it wasn’t much to look at when it debuted in 1993. Some days it was on “volume life support,” trading as little as 18,000 shares; there was even talk of pulling the plug. Yet true believers,...