This system would NOT be realistic, and the gains would be only to thieves. How realistic is it that impenetrable buildings should have chests inside them that can be broken into? Not really at all.
The lock system only benefits nomads, and even then they can be picked. In the end money is just wasted by everybody involved.
If I live under protection, who's to say that I can't keep my chests in a 3-block thick cobble room underground?
That's as realistic as any vault in the medieval era, it's only inconvenient.
If we around allowed to cover our chests with blocks directly, we're gonna surround them anyway. That itself is a ridiculous, counter-productive rule.