Gee, the government might not be able to force quite as much money from the citizens, and would therefore have less money to squander on idiotic crap nobody wants, needs, or asked for. That'd be a real crying shame.
If everyone paid based "only" on what the government knows about their income, the government would still have plenty of money. If they're hurting for cash that bad, they should take a big red pen,
go through this list, and eliminate 50% of it.
Golden Field Office. Inter-American Foundation. Japan-United States Friendship Commission. Management Assistance Team, Management Service Office. National Wild Horse and Burro Program. Center for the Book. Citizens' Stamp Advisory Committee. Executive Office for Weed and Seed. Office of Faith-Based and Community Initiatives. Tribal Affairs Office. Federal Duck Stamp Office. Six agencies, that I can see, doing the same exact job as the FDA. It goes on but I'm tired of looking at this list.
In short, I have no problem if the government gets less money. Who cares? You're also discounting the lost productivity and contribution to the GNP by having the entire labor force of the country throw away hours or days of their time filling out useless paperwork to tell the government what it probably already knows. I have no numbers but it seems to be the odds are pretty good that the act of filling out tax returns is vastly more expensive to the nation as a whole than a couple of people failing to report their two-thousand-dollar investment earnings.