This really depends on which city you're in and where in the city you leave your bike+helmet.

I've had the ultra-cheap lights on my bike stolen. I've seen plenty of other bikes stripped of everything except the frame itself.

This is why I always take my helmet (and my lights) with me, lock my bike and the front wheel with a good lock, and chain lock my seat down for good measure. Haven't had anything stolen since -- even in some sketchy areas.

A broken strap isn't a broken helmet. Anything not bolted to a bike gets stolen, including lights and seats.

It's just kind of a privileged thing to believe. Many people don't live in places they can safely leave their possessions alone in public.

And even if the actual rate of crime is lower than the perception, it's still an anxiety people have to add to their daily routine.

I agree with your presentation - the issue boils down to it not mattering how easy it is, it's the perception of how easy it is. Once people see they can leave their helmet with the bike and it won't get messed with or stolen then it can become normalised and the perception falls away. But the inhibition is in the perception that requiring a helmet makes it difficult.

Unrelated anecdote: the first time I went out with a helmet was the first time I banged my head, I ducked under a branch as I normally would, only I'd not allowed the extra room, banged my head hard and came off my bike (gently). I still wear and encourage helmet use.