I would worry about the value of employees who basically wanted to leave the company four years ago but are only hanging in because their shares are worth a million. Surely it would be better to get those people paid and then out the door rather than keeping your senior influential ranks full of people who stopped caring years ago.
Surely there is a better way?
Stock options are some kind of payment - so why not treat it as a pro rata accrument. You are the first hire - you get 2% of the company if you stay ten years. Leaving after five to get married and move country? Fine here is 1%, just sign here, and we all are happy.
No matter how nice your arresting officer is, everyone resents handcuffs.