I city shop. Unfortunately I like to go into many shops... to shop. I could do this with a Brompton, but many shops have only stairs, and its sorta crowded in my city. I tend to think walking and shopping without a Brompton is somewhat easier than with one. I could be wrong
Agreed. At least, as long as there's somewhere decent to park your bike.
I also rend to think a locked bicycle is more attractive to steal than a step thru... even tho my step thru is a pretty decent bike. Again, I could be wrong.
I'm not sure what frame geometry has to do with whether you lock your bike or not. Do you consider your step-through bike so unattractive that you don't bother locking it?
I liken it to the days when car radios were stolen. Once China made many and the prices fell, the theft in radios disappeared.
Over the same period car radios became orders of magnitude more useless once stolen. They developed security codes, removable front panels and eventually pretty much every car had a factory-fitted entertainment system that couldn't be removed without dismantling the dashboard, and wouldn't be much use to anyone who didn't have an uncommon fault in an identical car if you did.
I don't think there's a reasonable cycle-security analogy, beyond e-bike systems that are effectively bricked if you don't have the detachable console. And a bike isn't analogous to a car radio; it's analogous to a car. Steal a non-functional e-bike (or car) and you've still got a decent collection of parts to sell.
Anyway, you seem to like the idea of a Brompton, without having a need for a Brompton. Which is fair enough, by all means lust after the shiny toy, maybe even buy one to play with. But it sounds like you've already got an excellent bike for going to the shops on, so why would you want to use a Brompton for that?