"Product that was contaminated or could potentially have been contaminated must be destroyed"
That is mainly where my question lies - what would define "potentially have been contaminated?" If you have full confidence in your MD sensitivity then shouldn't you only destroy product that did in fact have metal in it?
That's why, in my opinion, root cause is so important. If you determine that the metal came from a supplier, for example, maybe it's an agricultural product and there was a metal machine piece from the field - that's not necessarily likely to be an ongoing food safety risk or contain many pieces. So then maybe it's just throw away the bag / product that was rejected by the metal detector.
But what if it's metal on metal wear on your line. In that case it's not appropriate to say 'oh well, our metal detector will catch it' because you can't guarantee that some of the metal shavings will not be below your detection threshold, not to mention taking an insane risk that your metal detectors will not fail.
Imagine that your kids or family members choked on / got cut on a piece of metal in something they ate and you wrote the company and they responded "we knew there was metal, but our metal detector should've isolated it all". How is that going to make you feel?
I'm personally not comfortable saying anything less than "I did everything I could to ensure only safe product went out" and if that means throwing away the upstream product until the step that introduced the metal, so be it.