Responding to the grandparent's edit.

I disagree with your premise that a vesting cliff exists as a "getting to know you" period. It is a guarantee of a minimum investment of time for a given amount of equity. The person put in the required time and now wants to do something else. This is not dishonorable in the least.

You seem to be confusing personal relationships with business relationships. This might explain why you see quitting as a sort of betrayal.