Just as no one knows at what rate it the 401k be taxed at when you begin to take it out, I can see the govt. down the road say there is a 'crisis' with SS, and Roth IRA's are no longer tax free.
You are exactly right about SS and Roth IRAs being taxed.. Anyone who thinks that Roth IRAs will be "Tax Free" is crazy. Congress will change Roth IRAs some time in the next 10 years.
Here is a fun fact for you:
If you hit 72 years and do not need the 401-K money, it does not matter. The Feds will tax you at 50% of their imputed required annual withdrawal.
Neat eh?