What would be acceptable evidence for you?
I left out a bunch because my note was long AF.
When we transfected cells and could see editing via next gen sequencing, we had controls. If we 'transfected' with just buffer (no AAV vector) or if we transfected with purified empty capsids (no payload) or if we transfected with a non-functional 'dummy' payload we got no editing. So we controlled for some spontaneous process occurring in the cells to give us our results - it turns out we required intact capsids with the correct engineered payload to see function that we could measure.
For your car analogy, did you also park your car under a carport roof or next to a sprinkler on a sunny day to explore other ways a car could get wet or stay dry?
An AAV virus is on the order of 25 nm in size, so hard to study. Like many things at this scale, we must be creative and attempt to form hypotheses and devise experiments to test them.
If I had access to a BSL4 lab and a supply of monkeys recently deceased from ebola and I handed you a teaspoon full of their bodily fluids that I had subjected to techniques used to purify/enrich for viral particles, would you consume it if I offered you 1 BTC to do so?
[ spoiler: I would not let you because I care about your well being! ]
Humans may think we are pretty smart monkeys, but there are innumerable things in our universe that we do not understand or even know about. I find being comfortable with this more fun that constraining what I am willing to ponder based on an evolving framework.