Once you start targeting public market holdings, it would just create an even stronger incentive for companies to remain private. Soon enough you'd have secondary markets for high rollers dealing in shares of private entities, which would be completely inaccessible to the average Joe - and congratulations, you've increased inequality. In fact, this is already the situation in technology investments to a large extent. Companies remain private for longer to avoid post IPO regulations and restrictions, and the only guys who can invest are the insiders who get the opportunity through their network. The companies are only pumped to the public when the ascent in value has pretty much diminished (SNAP, anyone?).