SAN FRANCISCO — Endless chatter over unicorns, and their stratospheric valuations, has added a layer of uncertainty to the usual mix of anxiety and excitement over disruptive technology.

Alongside the preening and pontificating by the tech stars gathered at the Fortune Global Forum, there was this persistent worry: This booming tech market is near a top, and both traditional companies and their high-tech disruptors are in for a squeeze. In particular, there are jitters over Unicorns, privately held start-ups with market valuations of at least $1 billion.

Unicorns “are waiting too long to go public and raising too much money,” Salesforce CEO Marc Benioff said. “They’re missing out on the public experience” of an IPO.

“It’s a good sign these companies (Unicorns) are thriving,” Yahoo CEO Marissa Mayer countered. “It’s a lot different from 1999 (shortly before a dot-com meltdown eviscerated many start-ups). Today’s companies have real businesses.”

These were among those gathered at San Francisco’s opulent Fairmont Hotel Tuesday, and a high-powered assembly that included Facebook Chief Operating Officer Sheryl Sandberg, venture capitalist Marc Andreessen and Alphabet CEO Larry Page.

Not all Unicorns are anxiety producing. Car-sharing service Uber is reinventing the transportation field and producing gobs of revenue.

Indeed, the most vulnerable companies may be non-tech companies. Uber, Airbnb and others have seen to that.

At the conference’s first day Monday, Cisco Systems Executive Chairman John Chambers, an unabashed optimist who recently retired as CEO after 20 years at the helm, ominously predicted 40% of the companies assembled here (tech and non-tech) will not exist in a decade. He said many will be victims of “digitization,” the influence of blur-fast digital technology in nearly every facet of all businesses. And, yes, Cisco is not exempt from the omnipresent threat, he acknowledged.

Mike McNamara, CEO of Flex, a design-and-manufacturing company, said “intelligent-end devices” (think digital sensors) and 3-D printing are fundamentally altering the manufacturing, automotive and medical industries. “It has changed the way we work, live and play,” he said.

Andreessen and Michael Dell, who was speaking overseas at the Web Summit in Dublin on Tuesday, discounted an overheated tech bubble about to pop.

“We’re not in a bubble; we’re in a bust,” said Andreessen, who is on the board of directors for Facebook and Hewlett Packard Enterprise. “Technology is undervalued, though. The entire basket of Unicorns is worth half of Microsoft.”

“People are excited in the valley. People outside the valley are still depressed,” Andreessen continued. “The stock market just doesn’t like tech.”

To be sure, there are “massive seismic shifts happening” in the tech sector as elsewhere, said Matthew Prince, CEO of security company CloudFare. Mobile technology made it possible for Uber and Airbnb to upend the taxi and hotel industries, and the power of smartphones are likely to create “a whole bunch of new tech giants,” he said.

“Mobile has reached 2 billion (people) in 10 years,” Sandberg said. “We’ve seen nothing like it.”

Put another way, Andreessen said, people are adopting mobile technology faster than electricity and water. He said 100% of his $4 billion VC firms’ investments are tied in some way to mobile.

Monday night, Google co-founder Larry Page, in a rare public appearance, explained the origin of Alphabet (Google co-founder Sergey Brin came up with the name) and vowed it will jump-start innovation at the search giant. “I think my job is to create a scale that we haven’t quite seen from other companies,” said Page, who is Alphabet CEO.

The volatility of the tech market was on full display at the three-day conference, which started in 1995 and hosted 350 executives from 32 Fortune 500 companies this year. It’s the first time Fortune magazine has held the event in California.

“We have a critical mass of people who want to take tech and affect change,” Mayer said. “That’s what brings us here.”

Follow USA TODAY San Francisco Bureau Chief Jon Swartz on Twitter: @jswartz.

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