Unrealized capital gains are not “cash piles.” I gut feel is if the government tried to tax them, it would drive a massive sell-off that would crush the stock and bond markets.
The Fed would prop up the stock market.
Look at the pension funds for IL, NY, and CA. They are all underfunded and all very heavily depend on stock investments. A market crash would make them drastically underfunded. Probably requiring tax increases in already heavily-taxed, predominately RAT, states.