I think it's reasonable to say "we have done enough" at some point though. We can debate where the point is, but safety is not an unalloyed good. It has a cost, and reasonable people can disagree over whether a particular safety invention has enough ROI to justify its existence.

For example, we wouldn't countenance banning all motor vehicles even though we could eliminate all car related deaths with that one simple trick. We would get a fair bit of payoff, but the cost would just be too high to justify it. Similarly, if we could inflict a very minor cost on everyone in the world to prevent one death per year, that would be too low of a payoff even though the cost is very low.

So yes, we can always add more layers of defense against accidents (or security incidents). But eventually, the juice isn't worth the squeeze and you stop. So I don't think the Swiss cheese model really can justify any particular intervention by itself; you have to evaluate the specifics of whether the particular intervention is worth it.