In addition to what other people mentioned (if you design something to have a 99.99% chance of lasting x years, it'll probably last some multiple of x), a lot of failures follow what's called a bathtub curve (visual depiction: https://en.m.wikipedia.org/wiki/Bathtub_curve#/media/File%3A...).

Once you can make something work for 1 day, you're past the most dangerous phase.