Well, if the law is set up in such a fashion that you can "keep receipts" and report retroactively to enjoy the full benefits, you would still have an incentive

Well, if the law is set up in such a fashion that you can "keep receipts" and report retroactively to enjoy the full benefits, you would still have an incentive to dredge everything up the moment you retire (and put the extra 4x lifetime bribe earnings into your retirement fund or bequeath them to your kids), and while the crackdown would be delayed, the bribing parties would still eventually come under the hammer (so unless there is some way for them to take and run with their short-term earnings leaving a shell company to be punished, there would not be an incentive for them to bribe in the first place).