Except that it gives our ever-more-intrusive federal government a peek at every single purchase made, which as the history of every federal tax system shows WILL be abused.
Upshot is that taxation sucks and is subject to take unconstitutional paths without consequence.
Maybe the better question is: why, after 225+ years of economic opportunity, the federal government isn't financially independent? and what could be done to make it so?
We can still use cash.
Upshot is that taxation sucks and is subject to take unconstitutional paths without consequence.
That the downside yes, and why we need courts of law with good lawyers and good judges.