Why stop at 5%? Why not 50, 100%? So let's assume people don't get the choice of "reducing" their taxes, but get to allocate on a per-budget category level how much of all their individual taxes get allocated? Someone can allocate the whole 100% of their salary to say education and 0% to military/defense spending. Another might decide to put lots into healthcare, plenty in education and none for military. Others would allocate 100% to military, etc.
Why would that be broken on a societal level, and how is it not "democratic" to allow people to allocate their "voluntarily given" taxes how they please? What are we afraid of happening? Can we not control the problem slightly and just give them high-level categories to try it out?
Well about half the people in the US don't pay any net income tax. So giving 100% of nothing towards the political goals they desire might be seen as unfair to some.
There are merits to such a system, but I suspect they don't align politically with what you intended.
Why would that be broken on a societal level, and how is it not "democratic" to allow people to allocate their "voluntarily given" taxes how they please? What are we afraid of happening? Can we not control the problem slightly and just give them high-level categories to try it out?