[fergus_google]

It's common for code at Google to be rewritten without reusing most of the existing tests -- instead, new tests are written for the new code. Yes, this risks not understanding edge cases. But not all of those edge cases are still important.

[why_only_15]

Maybe a compromise would be that if you rewrite old code you individually go through old tests and have to sign off on deprecating them, and say edge case X is no longer important, so you keep most tests while not using ones that don't matter.