History is littered with the broken bank accounts of investors.
Market crashes occur once a generation, so most speculators can't remember what happened as far back as 1987, when the last correction hit markets. That's a dangerous philosophy in any economy. Speculation comes when popular imagination settles on something new in the field of commerce. Galbraith isn't talking about the Internet here, but a depression caused in 17th century Holland by speculation in tulip bulbs. Based on history, the market will come down.