He’s spot on about ending the fairy tale that FICA and medicare are not taxes and including them as part of the income tax structure. I dont know about equalizing rates regardless of source. Why take a risk of loss if you’re return is taxed at the same rate as salary?
Same reason we do today, expected value exceeds opportunity cost. Of course, raising tax rates will lower the expected value of a lot of investments enough to put them below the opportunity cost, all other things being equal.