Currently less than 4% of Americans save any money. To assume that any would save enough to compound instead of spend in the absence of taxes is naive.
So you can't argue that lack of savings under the current system implies no savings under a low-tax system. It is pretty reasonable to suggest that a big contributing factor to the lack of savings is precisely what the tax system does to savers.
* I think that the 4% number is dubious but it's not material to the question.