That logic seems a bit circular. I don't endorse the 90% figure because I haven't thought about it, but for the sake of argument if you were faced with a choice where 90% of your income would be redirected away from you vs not saving it'd be expected that nobody would save.

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.