What's to stop them from terminating the contract a month shy, because of "performance reasons"? Unfortunately this setup would require quite a lot of trust in the industry, which, quite frankly, it has not earned.
If I stayed at a place a few extra years in anticipation of an IPO, this would have me seeing red.
Then there is the startup model. Where you're given stock of varying grades.
Stock always felt like a gamble. Give me enough for bills + saving, then treat the stock as a bonus. Not core compensation.