Capital gains is a reduced rate, not a deduction, so it’s not really a question of “keeping” it. And you probably need something in there that takes family size into account — a flat $15,000 exemption won’t do it.
The purpose of a flat tax is that it’s the same, there is no differentiation in the rate paid for anybody.
If you re-invest your money, you have not drawn an income from it, so there isn’t anything to tax, which is what I mean.
When you change the tax system, somebody is going to take a hit, but the hit needs to be taken in order to effect a positive change in the end.
But I would be fine with deducting dependents.