If the supermarket starts spending more on labour/lost carts than they were losing with lost business/upfront cost, they may bring them back.

I wonder what the conversation is between supermarkets. They have to know the one spending upfront for these devices is training the local population reducing the other's running costs.

I'm in Germany too, nobody leaves their trolleys sitting around.

That sucks. I noticed our Lidl did the same (maybe other supermarkets, too, haven't paid attention), but people still return the carts. This is in Austria.