Used soap bars get dry and then crack. There are two options, throw away the remains or make a mini-soap-ball that will keep cracking if you don´t keep the pressure and humidity.

A simple shape change from the manufacturer, a dent in all soap bars, will allow new ones to be filled with the remains of used ones providing a helpful solution, ad infinitum.

I´ve proposed this idea to Unilever ↗ and they reject it; they know nothing!