Out of curiosity, why do caching DNS resolvers, such as the DNS resolver I run on my home network, not provide an option to retain last-known-good resolutions beyond the authority-provided time to live? In such a configuration, after the TTL expiration, the resolver would attempt to refresh from the authority/upstream provider, but if that attempt fails, the response would be a more graceful failure of returning a la…
i seem to remember that dns has generally been reliable (until recently, i guess), probably nobody has ever thought that to be necessary.
you could write a cron script that generates a date-stamped hosts file based on a list of your top-used domain names, and simply use that on your machine(s) if your dns ever goes down. that's basically a very simple local dns cache.
if you feel like living dangerously, have it update /etc/hosts directly.