The most impact I see would be on people for whom non-liquid assets comprise the majority or entirety of net worth. Take a YC unicorn for example. As soon as a 409a valuation puts the value of a founder's common stock above $50m (in Warren's plan), the company has to start thinking about a liquidity plan in order to support the payment of wealth taxes. Unintended consequences could be further concentration of ownership and upside in VC/PE hands, more pressure to IPO sooner, or decreased appetite for risk among founders once they've hit a certain threshold.