In Norway, the trolley coined gained popularity as society went mostly cashless, yet the trolleys demanded their token. An earlier factor was that it was annoying to make sure you always had a coin of the correct denomination (physical size). Trolley coins can be part of your keychain, or won't be accidentally used to buy a newspaper before going to the grocery store.

Most people still return the trolley and slide it in, like civilized humans should

Yep, in Poland we have those locked trolleys, but more and more stores don't do that anymore, as people return the carts anyway

Tbh I think the plastic coins came around more as a convenience. If you have one in your wallet you always have the option of taking the trolley. You might not always have coins or the right sized coins. At least I know I don't have any coins most of the time.