I don't know about Crocs but brazilian flip-flops "Havaianas" never lost market share even though it feels so trivial to clone it.

The only two modes of failure of this sandal is "drying up" after a couple years and broken thong straps (yes, 3rd parties sell those as replacements..)

I guess they make sure they have cheap offerings and are always investing in design and marketing so no one can enter this market..