Well boo hoo. All income should be treated with a flat rate. No special rates no special deductions.
Raising the tax on savings and capital gains will result in a big drop in tax revenue. Investors will simply leave their money where it is and, thus, fail to take the gain.
Not only does that reduce tax revenue, it makes the economy less efficient as money doesn't flow as quickly out of inefficient companies to newer, more efficient ones.
Your suggestion is great class warfare, but poor economics.
When a person invests in a company, that investment can lose value, even a 100% loss of principal. When a person puts his money in a bank his principal with interest is guaranteed. I don’t see why both of these “incomes” should be treated the same.