Good question. It hasn't empirically. See founders taking money off the table in secondary in series A, Airbnb etc.
Plus, financially derisked founders are paid a premium (those with previous exits) by VC.
So we know that this is mostly an academic argument, but it is a resasanoble concern.
Moreover, if 90-95% of your holdings are your company, you are still motivated to make it a success, despite the 5-10% diversification. And most great founders are motivated by more than pure financial upside.
