This is simpler but tends to favor the wealthy because such taxes do not impose the same burden proportionately. Moreover the poor rarely invest--I think 401k uptake is only 50% in the US?--and would in present circumstances be challenged to do so. Maybe incentives would change for them under a new system. Another difficulty is that investment income grows over time and governments stand to lose a great deal if they don't tax it.
1. some basic needs like food and clothing could be exempt from sales taxes, like it is today in many US states.
2. Taxes should only grow in relation to the growth of identifiable government needs. There is no reason that the government ought to grow linearly in proportion to the net wealth of the country.