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.

Thank you :)

A little more information, considering it is public. (I had to double check if it was)

https://www.google.com/patents/US8583801