Most people would continue recursively:

• shovel sharpeners
• shovel marketplaces
• shovel SaaS

This answer breaks recursion by returning to:

the underlying constraint field

Why “shoes” is structurally correct

Mining creates:

• movement
• abrasion
• fatigue
• recurring replacement cycles

Meaning:

the real invariant isn’t “shovels”
it’s human bodies interacting with terrain