I think there's a difference between allowing fraud for 4 years vs allowing it for the rest of the life (could be 40 years). Fraud could be caught during N-400, but it can become more difficult if somebody applies under 5 year rule. Plus another stage in immigration process is another review and another opportunity to catch fraud.
I lost the thread with I-751 stats, but don't forget the big numbers... Even with 2% denial rate for each 100000 cases we're talking about 2000 potential fraudsters. That's a small town! And if naturalized, they're going to use all the benefits this country has to offer. In a lot of cases they'll sponsor somebody else (spouse, kids, parents) meaning even more people piggibacking on that fraudster.