JFZ90 - Spot-On!
There is no leakproof or defect-proof system - only more leak resistant or more reliable systems.
Modifications come out all the time, due to the findings/reports of analysts and end-users, and each Airline vets whether the Mod is suitable or not for them - unless it is mandated.
Even before the Mandatory Mods are published, plans are (or should be) made, by the mandating authority and the OEM, to ensure the right parts are available for the affected fleets and the dates to comply with mandated mods are also mandated because they KNOW the availability of said parts. The risks in NOT embodying the Mods may involve the grounding of the fleets.
Some Mandated Mods are subjected to fleet planning and implementation times that may, in some way, risk an incident that is considered acceptable during that implementation time.
I (rather naively, I suppose) assume these risk values are very similar to those published in the CAA/ICAO SMS risk value charts and are, I suspect, "your" ALARP Plus items - on their way to being ALARP.
However; should I assume MOD would not be using CAA/ICAO values?