Patrick Collison observes that Mathematica takes backwards compatibility seriously, allowing programs written 20 years ago to run unchanged.
That really raises the stakes in API design for sort of obvious reasons. We have that problem ourselves, where, when we think about introducing something new, it's not just: "Does this exigently address the particular need that's motivating it today?" — but: "Do we think we can stand behind this in 2044? How do we think the world might evolve around us, such that it all remains coherent?"