It sounds like a good test in theory but I would say it's actually more nuanced than that.
In my experience, there are certainly reasons that returning the cart might be difficult or impossible (handicap, small children etc.).
If you speak to employees about it, I have been told that they often actually like going outside to get the carts, so to me this is not only increased convenience for me personally, but desired by the employees also as they get a "break" from the chaos inside the store.
I've worked retail jobs where employees were assigned lot duty on a rotating basis, and I can assure you most people didn't want to do it and staff had to be vigilant to make sure it wasn't being neglected. It's moderately hard physical labor (assuming there are no powered cart-pusher things and the lot is large and on some kind of slope), and is out in the elements where it can be frigidly cold and windy or swelteringly hot. Some employees might be misanthropic enough in context that they'd rather do it than work inside, but it's definitely not all or even a majority in my experience.
> there are certainly reasons that returning the cart might be difficult or impossible (handicap, small children etc.).
If you can get the cart around the store, across the parking lot and to your car, you can get it back to its home too. It didn't teleport to your car.
A loaded cart, no less.
>I have been told that they often actually like going outside to get the carts
They are getting the carts either way. No one is saying to return all carts to the store. There are several partking lot racks for a reason.
It seems to be different in the USA, but here they wouldn't. If somebody would abandon a shopping cart, it would stay there indefinitely until somebody returns it. I mean the employee might do it after work, on there way home, but also only for the same reason any other random person would do it.
> if you speak to employees about it, I have been told that they often actually like going outside to get the carts
This has been my experience as well, but people will always blindly insist the opposite just to "win" the shopping cart argument.
The employees arent there to have fun. When customers create work for the store they need to spend more on wages and prices go up. The shopping cart theory isnt about the employees, its about making the whole system a little bit worse for everyone else.