Aren't fire hydrant valves usually a few feet underground, which makes running them down not as exciting as it could be?

Gas pumps (gasoline), at least in Canada, the pump is underground and you're really just holding a big nozzle. Bad, but not too bad. People have been driving into and through those for years.

Natural gas lines and meters could be a catastrophe, but they're usually pretty close to a wall.

Those don't move, so memorizing their location/occupancy should work better.

How much object persistence does this system actually have? Watching objects jitter around the screen and morph from a sign to a cone to a person back to a sign hasn't inspired confidence in me.

For sure, but it's not in the same category as software that is making safety critical decisions at highway speeds. Risk is usually severity x probability. I imagine the probability of death in a parking scenario is much, much lower than other driving scenarios. Not zero, but pretty low.