Another toenail goes to the toenail in the sky.  Not sure why the sandals eat toenails.  The general theory is bare EVA is very sticky & too much stickiness in the toe area causes blisters under the toenails.  You can't moisturize toes because moisturizer causes toes to stick.  Simultaneously, dry toes get cracked & swollen which also kills toenails.

 

The leading idea nowadays is to lubricate the toe area but not use a moisturizer.  Another idea has been to cover the bare EVA with suede, but suede is heavy.  Suede has provided just enough stick but not too much & it's been immune to rain.