I think I understand what you are saying but I don't believe its accurate.
Lets keep things simple. Assume an initial $5,500 investment (Post Tax), 6% ROR (compound), 28% tax rate, no annual contributions just the initial lump (again for simplicity), 30 yrs.
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401k (taxed when you retrieve the money)
$5,500 * 1.28 (since 401k is pretax) = $7,040 initial investment.
6% ROR @ 30 years compounding = $40,434.18
Then you get taxed at 28% = $29,112.60
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Roth IRA (taxed pre-investment, not taxed later)
$5,500 @ 6% ROR @ 30 years compounding = $31,589.
My math may not be right. Please correct me.
Plus then you have the added benefit of being able to use your initial investment if you got into an emergency and needed it. Do I think Retirees will get taxed at 40% by the time I retire I sure hope not. But one thing I am pretty certain of, is taxes will not go down.