Slack, a company that's been around in its present form for less than 18 months, recently raised $160 million at a valuation of nearly $3 billion. So its founder, Stewart Butterfield, is more aware than most of us that tech company valuations might be in a bubble.

At the Collision Conference in Las Vegas on Tuesday, Butterfield noted that he has lived through multiple downturns, including the 1991 recession, the dotcom crash of 2001 and the financial crisis of 2008. "I'm conscious there's a cycle," he said during a mainstage interview with Jonathan Krim of The Wall Street Journal. "It's very difficult to say when you're about to fall, but it's easy to say we're very far from the bottom."

That's not the same thing as saying we're due for a crash, however, he pointed out. "I think what people often have in mind when they say there's a bubble is that if there's a bubble, you can pop it and then there's nothing." He doesn't see that happening in today's Silicon Valley, even if rising interest rates or some other shock to the system should cause a pullback of investment. "Does it go to zero? No," he said. "Is Uber worth $20 billion instead of $40 billion? It's a possibility, but there's real revenue there."

Butterfield's more immediate fear is that the runaway growth of Slack, which makes workplace collaboration software, will force him to grow his workforce faster than he can evolve his management methods, making for an inefficiently, unhappy workforce. "We had one day a couple weeks ago where 14 people started," he said. "At that point, more than 10 percent of the company had their first day that Monday."

He's also aware that big enterprise-software players, especially Microsoft, are looking at what Slack has done and plotting how to steal a piece of it. Butterfield said he's moderately worried that "someone figures out a way to deliver 80% of the value Slack does with 20 percent the complexity." 

But that, he said, is more motivation for working hard than a genuine fear. "I say bring it on. It's funner to compete against competitors."