FWIW, when I lived in North Dakota, we actually had a state tax return which could be filed on a post card. Compliance was pretty good as well.
How would you know without some agency checking it?
Great idea...except if the guy has a business. Or do you want a business “post card” as well? Or if the guy has 4 rentals. Or do you want a rental “post card” as well? Or if the guy has a farm. Or do you want a farm “post card” as well? Or if the guy trades stock. Or do you want a stock trading “post card” as well? Or if the guy divorced his wife and pays alimony. Or do you want a divorcee “post card” as well? Who would read your post cards? Wait...we already have this: Called the IRS.