Why would I engage in an argument premised on how unreasonable the argument is? You said: "you couldn't have meant input validation is useless". You're right.
Besides, it's not just about us: other people, (including @carty76ers apparently) would like to know what you would advise instead of "please validate all inputs".
The reason I kept guessing, was because you kept not telling. Until then: https://news.ycombinator.com/item?id=21719065
Finally something I can argue with.