"Nudging" people to overconsume when they would otherwise save their money - which they need for their future, as evidenced by the fact that they'd save it otherwise - systematically encourages everyone to do the same thing with their money and overextend themselves all at once instead of letting their decisions reflect individual differences and balance each other out to find equilibrium. This creates an unsustainable bump in consumption, which encourages a spike of malinvestment in industry sectors which specifically service this cyclical jump in spending...thereby leading to a "cluster of errors," as the Austrians put it. Moreover, penalizing people for holding onto their money encourages investors to make rash investment decisions to avoid the penalty. You cannot play God with savings vs. spending incentives from a position of central authority and expect to do anything other than cause downard spirals and cyclical disturbances. There has to be a market-driven balance of savings and investment, and people need to be able to wait for the right opportunities to invest instead of being encouraged to throw their money around.
In short, yes, it WOULD be encouraging malinvestment...just like the inflation tax does today. If a non-inflationary tax somehow encouraged spending the same way without expanding the money supply, then there would be a finite limit to the malinvestment instead of letting it get so out of control, but it would still be present and cause problems of the same nature, if not the same severity.
Be careful what you wish for. The economy doesn't need people to be strong-armed into saving OR spending. Either one causes terrible economic imbalances.
