I'd stay away from "arches have keystones" argument if I were you. I see no keystone:
Would you:
A. Think it was the result of an unbelievably improbable statistical anomaly of nature.
B. Think someone designed it.
That was the simple question put out there. A simple question to test whether the reader had the willingness to change their mind in the face of overwhelming evidence, if such evidence were presented.
So far, the level of obfuscation in response points to a closed mindedness on the subject. It is impossible to sway people with mere logic, when they have proved themselves capable of rejecting things regardless of facts.
People who won't answer a clear hypothetical are generally insecure, but rabidly devout in their positions. No sense arguing with them, or in this case even pursuing with the test.