Theist "You are Evidence" argument.

[Esquilax]

("There is nothing new to be discovered in physics now. All that remains is more and more precise measurement.")
- by Baron William Thomson Kelvin, 1900, in an Address at the British Association for the Advancement of Science

This statement was made before the entire fields of relativity and quantum theory were even imagined, which flipped physics on its head.

The claim that we know everything that there is about a good biological design is equally absurd. There is huge amounts of biological information that we can't even begin to understand. So the idea of critiquing a biological design is child's play.

Its unbelievable how arrogant we can be as human beings about how smart we are.

I love this weird, inverse argument from ignorance, because it's so easy to debunk. While I agree it's no good to assume complete knowledge of a given field at any point, I also must point out that we can only form our conclusions based in the data available at the present time. Saying "yeah, this looks like bad design, but we don't know, maybe there's a good justification!" is exactly as fallacious as your Kelvin quote there: Kelvin assumes there's nothing more to discover, and you're assuming that there is a specific thing to discover. You're both doing so on the back of precisely zero positive evidence for those propositions, which is what makes them irrational.