100% reliability is physically impossible. "reliability" is defined to include resistance to abnormal situations. A bridge might be 100% reliable when winds don

100% reliability is physically impossible. "reliability" is defined to include resistance to abnormal situations. A bridge might be 100% reliable when winds don't exceed 100mph...an event that might have a 1% probability per year in its area of construction, which is clearly highly unlikely, but still something that is accounted for in reliability, uptime, and failure rate metrics. I don't even know if you can achieve 100% reliability given a set of plausible situations/usage, because you'd have to imagine every single possible failure mode for the thing that you're building. That is - the threat of "destruction" proper (which you contrasted with "collapse without prior warning") is included in the reliability metrics for a device. The point stands. Nothing that humans build can be 100% reliable - the only thing you can do is asymptotically approach it. Now, that said - three nines per year is way too low for me, personally. Five nines is more comfortable (and if it's cheap I'd like to go higher).