Working at most startups (in their current form) in a world full of growing tech giants like Google, Facebook, Apple, Netflix, etc. is making less and less sense for most people who qualify for these jobs
-- Silicon Valley engineers are starting to doubt the wisdom of working at startups
The IPO window is shifting from a range of six to eight years to 10 to 12 years, which is one reason a string of startups went public at valuations over $10 billion this year.
-- Silicon Valley loses some entrepreneurial spirit as venture capitalists favor later-stage deals
The "circle of life" in Silicon Valley has always been that the reward for taking a low salary at a startup is a handsome payout when that startup exits, either via IPO or getting bought-out. This works for both parties; the startup gets access to the best engineering talent for a relatively small amount of capital, and engineers get to continuously work on cutting-edge technology, moving from startup to startup while still make a lot of money.
This formula falls apart when the engineering talent questions the viability of a near-term exit strategy. The value of spending even six years at a startup is small compared to the full wages and benefits offered by big tech, if there is no pot-of-gold at the end of the rainbow. The big tech stocks are already liquid and they have done very well, recently.