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. It's just a matter of when. Or the crash could drag out over an eight-year period that mirrors the current bull market.