Two of the highest-flying now-public enterprise companies of the year — Slack and Zoom — are different in many ways, besides the fact that one is focused on workplace messaging while the other is centered around video conferencing.

Slack began life as a very different startup; Zoom founder Eric Yuan knew from the outset that he wanted to take on his former employer, WebEx. Slack raised a lot of money from many sources before hitting the public market — roughly $1.4 billion over 10 rounds; Zoom raised $160 million across five rounds, including a $100 million Series D round funded entirely by Sequoia. The two also approached their public offerings differently. Slack chose a direct listing that didn’t raise new money for the company; Zoom chose a traditional IPO, raising half a billion dollars in funding for its coffers just ahead of its first day of trading.

Still, the two companies also have much in common. Both took on incumbents (WebEx and email, respectively). Both are rooted in workplace collaboration and, as such, have some of the same competitors, including Microsoft Teams.

As Zoom investor Gordon Ritter of Emergence Capital Partners also notes, both are “powered by viral end-user adoption, which is not the case for every SaaS company.” (Slack largely grows within a company, starting with one team; Zoom grows internally and externally, given the nature of video conferencing across companies.)

Perhaps more meaningfully, both may see fewer days at the top of the heap than some of their predecessors. The reason, as says longtime VC Greg Gretsch, who co-founded Jackson Square Ventures in 2011, “The intensity of new competition is on a completely different level today from what it was 15 or 20 years ago.”

Put simply, the “cycle time of incumbents having their day in the sun is getting shorter and shorter,” adds RItter, who co-founded Emergence in 2002 and has backed Box, Yammer and Veeva Systems, where he remains board chair.

It’s happening broadly to Fortune 500 companies, whose average lifespan is now less than 20 years, compared with 60 years in the 1950s. Now, even fast-growing companies like Zoom and Slack, which “have amazing futures,” says Ritter, will likely have startups nipping at their heels very soon.

Craig Hanson, a general partner and co-founder of NextWorld Capital in San Francisco, explains it this way: “In the current environment, with all the entrepreneurs and capital looking for the next great idea, each startup success story immediately blooms an entire field of new startups chasing after them.”

It’s almost possible to time it says Hanson. “Once a startup raises a big growth round or has an impressive exit, in two to three quarters, you’ll see rounds of funding for similar new companies. This happens in both consumer and enterprise tech. VCs may regret missing out on the first company that just raised big and hope that there’s room for another one, or some great IPO or acquisition may spark a newfound passion for a space they overlooked before or that they thought was too hard until someone proved them wrong.”

Consider the many failed video conferencing startups to precede Zoom, including TeamSlide, LiveMeeting and Vyew among them. Eric Yuan’s startup was anything but a sure thing, But once a space has been validated by the kind of success it’s enjoying, it makes it easier for founders to raise money. This might partly explain why, in April, a nearly five-year-old, Boston-based startup named Owl Labs raised $15 million in Series B funding for its video conference camera with 360-degree capabilities. Another web conferencing startup, Highfive, based in Redwood City, Calif., raised $32 million last year, including from Lightspeed Venture Partners, General Catalyst and Andreessen Horowitz.

“It’s easier to explain what they want to do [if they can say] ‘We’re like Twilio for ____,'” says Hanson, who says that as recently as 2016, “you’d have maybe two to three startups going after a space and chasing the incumbents. Now there will be 10 startups, and the incumbents were themselves startups just a handful of years earlier.”

The trend isn’t limited to recently public companies, adds Gretsch, noting that “success for many companies and sectors is declared long before the first IPO, and with that perceived success comes a wave of me-too competitors.” It goes “hand in hand with the explosion of seed rounds over the last 10 years, which itself has been largely driven by how little it really costs for a company to get a finished product into customers hands,” he says.

“Now when a new upstart company starts to get traction, there are almost immediately a handful — or more –of startup competitors. Think of the massive proliferation of on-demand startups that emerged in the wake of Uber’s early traction — many of them direct competitors to Uber’s core service.”

Gretsch isn’t so sure the trend is a new one, he says. Nevertheless, because the sheer number of startups that receive funding to put out new products is “off the charts,” it’s changing the game for consumer and enterprise companies alike.

“Any company that’s enjoying success has to remain paranoid and not ever settle for resting on their laurels,” says Gretsch. Now, it just happens to be “more true than it was 23 years ago, when [famed Intel CEO] Andy Grove used it for the title of his book.”

That book: “Only the Paranoid Survive.”