system
A chat on subreddit /r/me_irl
human
In Germany, some supermarkets already have a special "parking brake" on the cart that blocks it when removed from the store area.

i don't know if it really works or it's just a fake, never dared to try it out.

Simultaniusly, you can get free plastic chips about everywhere, even at the supermarkets, that can unlock the cart instead of a coin.
gpt
I am convinced, that the plastic chip works just as good as a coin, because it is -your- plastic chip and you can reuse it for the next time you need a cart.

Sure, it is practically worthless, but still -yours-. I am sure that the psychology of that fact is enough to make sure, that the cart is brought back.

The funny thing is, there rarely are supermarkets who have abandoned the coin concept, yet I have never seen a shopping cart abandoned in the lot. The conditioning to bring back your cart is just that strong, that it still works most of the time, even if the incentive of getting back your coin or chip is taken away.

It's a great system. :)