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.

Only 1-5% of founder's shares are contributed. 95% or more retained. It's insurance; not a plan B.