Mens rea is an important consideration so it's not just about damage done (though the fear a key could be used in pursuit of a worse crime is also a harm) but the intent/recklessness of an act.

Well, the imaginary perfect criminal justice system would probably arrest you right as you had completely committed to causing the damage, instead of afterwards. But it should still be justifying the arrest based on the act that caused damage, not the harmless acts that set you up to be ready to do it.

Ah, like Minority Report

The crime in this hypothetical degrades from Burglary to Trespass, not "no crime."

Trespass is not a crime per se. It's a tortious act. Criminal trespassing is when you commit trespassing when specifically told to leave and you don't or do it again.

Lesser punishment doesn't mean no punishment. Furthermore, you can always argue that the intent is to commit a major crime.

A burglar might kill someone, book them on home invasion charges even if the house was empty.