The economy wouldn't collapse, but the tax income could probably suffer. They don't expect to collect very much out of it even now - see the link above in the comments - it's largely a populist gesture. But the tax-avoiding measures that high-earners would take to avoid it will stay even after the tax will be lowered back (provided it would be as promised), so the next effect may turn out to be even negative. But it would hardly lead economy as large as France's to collapse. As for if it would make the appetite for electing politicians prone to useless populist gestures instead of solving real economic problems - that is harder to predict if it would increase or decrease.