That's assuming people have any idea how many audits the IRS is doing, or will do at any point in the next few years/administrations, and are taking it into account when deciding whether to commit tax fraud. People don't work that way.
Also, they haven't actually stopped doing audits, so enjoy playing Russian Roulette with three bullets in the chamber. Still doesn't sound like a good idea even if there used to be four, does it?
Sure, people don't know the actual number, but they know that some enforcement is done, and some enforcement must be done for real for the effect to ripple through society. If you see your neighbour get away with it, it does entice you. The easy counterexample is Greece, where enforcement is notoriously low.
