It's not complicated. If the company is that bad that people want to leave as soon as they're vested, then the company deserves to fail.
So you think the managers should just look around, say "yup, we deserve to fail" and give up?
No. That's not the way it works.
I'm saying they shouldn't consider a solution being something that restricts they're employees. They should be looking at why people want to leave, instead of how they can prevent people from leaving.
I'm operating under the assumption that most people aren't counting down the seconds until they can leave, but some will want to. Four years is a long time, and 10+ years to IPO is a quarter of your career.
And if one person leaves it's not likely to materially affect the business as everyone else keeps it going.
Another point of view is that if all the early employees disappear at the 4 year mark (or whenever they feel they've vested "enough") that could cause very serious problems for the business. There is an element of a prisoner's dilemma here and it's not unreasonable to think about ways to keep people from defecting.
As I said, it's complicated.