I own a Miata and I've heard all the jokes, wrote many of them myself.
People who own the earlier cars for a long time eventually develop a routine for keeping everything safe.
In a Miata, as in any car, you don't want leave stuff unattended, but it goes beyond that. It's *really* easy to break into an open car with a folding fabric roof and no security system. A box cutter will cut a nice sized hole in the top making it easy to reach in and unlock. So even if you have nothing in the car worth anything, a guy on a streak of car crimes is probably still going to trash your top just to see what's in the car.
Therefore, I never lock the car, or the storage areas in it. The most valuable thing anyone is likely to "take" (by virtue of destroying it) is my top. Those are like $500+ and installation not being fun.
People probably *have* been in my car, they've just never stolen anything because nothing was in it. This is maybe a lesson for some far thinking regarding possessions, the idea of worth, crime, etc but I'll let you figure that out.
Regarding the car itself, with a racing seat, a rollbar, a hardtop, and a stick shift its probably too hard for half the population to even fit in in 2017 and chances are they can't drive a standard shift anyway. The best security system there is.