This seems like an over simplification which ignores people’s abilities to still draw cash on their illiquid stock. It’s not hard to imagine that early employees with millions in stock can take a loan against them, or they can sell their shares on the secondary markets too. This is especially true for Stripe.

The idea of taxing only realized gains is why we have problems like prop 13 in California with two neighbors paying radically different property taxes on the same valued homes.