Consider Cisco. On the 31st of March, 2000, it was valued at US$77.31 / share, which in inflation adjusted terms is $150.46 (above the current price over 26 years later). This valuation was on the basis of speculation that the price would continue to go up and Cisco would get a large cut of the industry profits. Cisco's business is still valuable, it was just treated as overvalued by the market.

Similarly, if we go back to one of the classic examples of a bubble - consider Dutch tulip prices in 1636; speculation drove future contracts high. But tulips still have value to people today, it's just the price was higher than was sustainable.