When Silicon Valley venture capitalists get themselves into image trouble, it’s usually because they’ve been too candid with their low estimate of other people’s intelligence. For example, after India recently prevented Facebook from offering free Internet service there, Marc Andreessen suggested in a tweet that the subcontinent might be better off had it remained under colonial administration. It’s much less common for V.C.s to soil their own nests in public. So it came as something of a surprise when Chamath Palihapitiya — Sri Lankan war refugee, early Facebook employee, investor in Slack and Box, part owner of the Golden State Warriors — told Vanity Fair in March that if we are in fact in the early stages of a second tech collapse, venture capitalists have only their own mediocre, clubby selves to blame. They should, he said, “focus on using capital as a way to take really big bets on things that just seem totally audacious. Right now we haven’t done enough of that, and the result is that most of the things we’ve funded are mostly crap and largely worthless.”

It was a striking admission. This “largely worthless” start-up scene, according to the research firm CB Insights, has raised an estimated $238 billion over the last five years — a remarkable bull run in private technology stocks. Forbes reports that there are now close to 200 “unicorns,” to use the Valley term of art for private companies worth more than a billion dollars on paper. Since the financial crisis, these companies, along with their more established public predecessors, have been seen by many Americans as the last redoubt of confidence and productivity in an otherwise uneven recovery. V.C.s have spent years dismissing speculation about a private-equity bubble as merely an expression, by know-nothing spectators, of resentment and alarmism; media onlookers, they argue, should talk instead about the triumphal progress of the genuinely great start-ups as they try to solve our most difficult problems. Over the past year, however, as allegations of mismanagement and unsustainability have grown — Square went public for approximately half its last private valuation; Fidelity and other large mutual funds wrote down their positions in Dropbox, MongoDB, and Snapchat; and both Zenefits and Theranos were accused of deceptive practices — that confidence has come to look more like hubris.