Consider: you write perfect code, and you write the perfect web and mobile app.
1. Does it have a server?
2. What happens when everyone upgrades their browser?
3. What if everyone upgrades their iOS?
It is not your fault that it may not work in 2 years, let alone 15.
1. It is good software
2. It looks the same as it did 10 years ago
Although I don't know for sure, I suspect it has been updated, modified, debugged and so on quite actively over the last 10 years.
Even if we assume that it were a perfect piece of software 10 years ago, craigslist, at some point within the past few years, updated its CSS to include @media rules to display properly on mobile browsers. This update needed to be made in order to support an entire class of devices that didn't exist 10 years ago.
Otherwise, consider emulators - games or applications written for platforms long gone can continue to run today, because that software was written (mostly) to some sort of standard.