Of course the guarantee isn't absolute; that would be silly. But so is thinking that this isn't an absolutely crucial distinguishing factor.

I can run programs written 17 years ago on my current Windows box, not even needing a VM. In computer terms, that's almost forever, and it was made possible by Microsoft's commitment to API compatibility.

API versioning is a mindset. It means that once you've published something, you can't change it without carefully managing the effects. It potentially means maintaining bug compatibility; it sometimes means detecting the client of the API, and specifically accommodating client bugs. Other APIs, even internal APIs used by rich web apps to communicate with their back ends, are not written with the same mindset. And rightly so; the constraints that come along with a public API are not to be taken lightly, and do hamper agility.

Code can last forever; there are people playing thirty-year-old games via emulation. The only problem is that we now have a plague of people who don't take their responsibilities seriously as contributors to the Web, and they randomly break their own resources out of laziness or ignorance.