Currently the tax rate is progressive. In the future it wouldn't be anymore. But because those progressive taxes only apply to income over a certain threshold, people with lower incomes would profit more.

This system would not replace social security. If you get a pension due to age or sickness or in your first year of unemployment, you would still be covered by your mandatory insurance. Same with your mandatory health insurance. And you'd still have to pay for it on top of your taxes. The employee and the employer pay 20% of the gross income each.