A couple of key issues I ran into:
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cost: to do anything interesting with mechanically interlocking (at least that I thought of) just made tooling so cost prohibitive that you might as well of bought several pairs of shoes.
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distribution: how do you sell the parts. It is relatively easy to sell in a shoe, but where do you sell parts of shoes. This is not a very good reason mind you, just a reality of the big footwear retailers.
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patience and persistence: to make sense the product needs to be in the market for an extend period. There is no use buying into a system if that system disappears in a season or two.
Nike Zvezdochka
The original concept here was to have a modular shoe with three main components, a rubber outsole the mechanically snap fit into a TPU foot form and an inner bootie. Each part could be traded out. The cost of the TPU mold was pretty insane… it was also the most beautiful mold I had ever seen!
There were a few follow on products that never really saw the light of day.