Does the alarm ever prevent theft?

If I’m living in an area where they don’t go off often (like right now) and a car alarm woke me up, I would definitely check.

And I imagine if I triggered a car alarm, I would back off.

Hard to prove empirically either way, I think. Even if they do, it's then arguable if this is worth the noise pollution etc. Probably depends on the circumstances in any case, so any empirical result would be too specific to be of any use. Science!

A thief can disable your blaring car alarm in like 30 seconds. Enough time for anyone listening to go "wow someone must have hit the panic button in their pocket" and move on with their life.

Car alarms are not useful for bystanders, they are useful for the owner of the car (who would know their alarm shouldn't be going off).

That said, this makes them near-useless in most public settings.

"That couldn't have been mine, someone already shut it off" They will think, as the thief disconnects their battery and loads the car on a flatbed.

Usually when they are going off in my neighborhood the car is streetparked and that's how they end up getting triggered (sometimes a loud motorcycle can even do it). So the owner could be like well over a 10 minute walk away and out of earshot anyhow. As a result most of the time when I hear a car alarm it rings until the thing shuts itself off automatically to save the car battery.