[homogeneous]

> A locker wouldn't have that problem, as there is nothing to break.

There is also no need for collateral or a smart contract in this scenario.

> Public bikes in many big cities don't have that problem either (i.e. there's no assessment of the returned bike).

How does a smart contract improve over the current system?

> A smart car doesn't have to have it either (sensors in the car could detect crashes, for example, and interiors could be built to be tough, like many public cars already - see Paris' AutoLib)

How does a smart contract help?

In all of your examples a smart contract doesn't add anything to the equation because the merchant (or the merchant's automated centralized systems) already have the authority and capability to relinquish deposited funds.