The assumption in the creation of the Service Bulletin is that you may or may not have a defective aircraft and if you do it may or may not fail in an unsafe manner in a given period of time.
If you exceed that time frame the probability of it failing increases to an unreasonable contribution in average risk compared to all other risks for both known and unknown problems.
There is always the underlying assumption that some un-inspected aircraft are free of the defect and/or that if the defect is present it will not fail and/if it does fail that it will not create an unsafe condition.
note: this kind of logic is way beyond "spectator balcony" stuff and suitable for the Safety forum