FWIW, since the comment was a reply to my message above:
It provided value by answering my question concerning serious downsides to providing optional post-TTL last-known-good caching within a DNS resolver. The answer is implicit in that a major DNS resolver provides exactly this functionality.
It provided value by answering my question concerning serious downsides to providing optional post-TTL last-known-good caching within a DNS resolver. The answer is implicit in that a major DNS resolver provides exactly this functionality.