This was supposed to be the year when America’s biggest start-ups would finally make their triumphant debut on the stock market.

Billionaire Silicon Valley investors, sneaker-clad founders and button-down bankers all expected enormous stock sales to turn companies like Uber, Lyft and WeWork into a new generation of corporate giants.

It hasn’t quite turned out that way. Last week, WeWork postponed its planned initial public offering. Uber and Lyft sold shares earlier this year only to see their prices collapse. On Thursday, Peloton joined the list as its shares slumped in their first day of trading. Investors took a look and backed away, seeing overpriced companies with no prospect of making money any time soon, in some cases led by untested executives.

The rejection threatens Silicon Valley’s favored approach to building companies. The formula relies on gobs of money from venture capitalists to paper over losses with the expectation that Wall Street investors will eventually buy shares and make everybody rich. If mutual funds and pension funds are no longer willing to buy once the companies go public, fledgling companies are unlikely to find funding in the first place.