Corporate leaders who backed President Joe Biden in the 2020 election conveyed deep skepticism that the so-called billionaire’s tax Biden proposed in his State of the Union address this week would ever become law.The plan would require households with a net worth above $100 million to pay a minimum annual tax of 20% on both their standard taxable income and on gains in the total value of their “tradable assets,” which includes stocks, bonds, mutual funds and other securities.Under current tax law, securities gains aren’t taxed until the owner sells them. Under Biden’s proposal, the ultra-wealthy would owe an annual tax of 20% on unrealized gains or losses in the value of those assets, whether or not they had actually pocketed that gain by selling them.The plan is …