Space Vehicles and programs are usually assessed and given a “Probability of Success”. NASA usually doesn’t launch anything without a Ps of 95%.

So if NASA has a program that is supposed to have a 2 year mission life, they would launch only when they have a design that has a 95% probability of success. Well if the spacecraft has a 95% chance of lasting 2 years, it’s not like it’s gonna crap out on 2 years and 1 day. It’s got a pretty good chance to keep going.