The dynamic that's pushed Slack's valuation to $2.8 billion in under two years is making it hard for the messaging start-up to put its cash to work.
Slack CEO Stewart Butterfield told CNBC he's looking hard at acquisitions after raising $280 million in six months, but he's not finding much value. He didn't name potential takeover targets, but smaller start-ups in the business messaging and collaboration space include Quip, Convo and Trello.
Read MoreSlack valuation more than doubles
"It's kind of like the minimum acquisition ... of a VC-backed tech company these days is 50 million bucks," Butterfield said in an interview on Thursday from the Code Conference in Rancho Palos Verdes, California. "That's kind of the floor, so it's expensive."
The number of private venture-backed companies valued at $1 billion or more has jumped to 96 from 56 a year ago, according to data from The Wall Street Journal. (Tweet This) Slack is ranked 31st based on valuation.
Now that the San Francisco-based start-up has pulled in so much cash, Butterfield says his company could really benefit from a dramatic cooling in the tech market. Venture investing in the U.S. surged 61 percent in 2014 to $48.4 billion, marking the biggest year since the dot-com bubble in 2000, according to the National Venture Capital Association.
"I wouldn't wish this upon the world, but kind of the best thing that could happen from our perspective is a giant crash," said Butterfield. "Office space becomes cheaper, salaries become cheaper, buying advertising becomes cheaper, acquiring companies becomes more realistic."
Butterfield admits that a major reason why he was able to bring in so much money at such a rich price is that all sorts of nontraditional investors are pouring into the tech market searching for growth. But he also says the business is doubling in size every 120 days.
In other words, investors are valuing Slack not on what it is, but what they think it can become.
"There's a bet on the size of the market and the ability of that team to execute against it."