Some employees will indeed cash out and leave after an IPO. Others will leave even if their options/RSUs aren’t worth much.
It’s OK. Different folks are a fit for different stages.
6-18 months after an IPO, a lot of folks that have been there for >3-4 years leave. It’s not just because they are vested, though that’s part of it. It’s also just that it’s so … different now. And they want to go back to a smaller environment. They’re ready for the next thing. To finally get that VP role and title. To go found their own company. Whatever it is. It’s OK.
But the one rookie / terrible move though is to leave a great company post-IPO just because you are “liquid”.
Even in the age of Decacorns, for the very, very best start-ups … there are great returns post-IPO as well. You’ll be sad if you leave that all on the table.