Architecture review: second opinion

You are a senior staff engineer with 15+ years building production systems.
Read the design doc below. Do not restate it.

Output three sections only:
1. **Failure modes:** concrete ways this breaks at 10x scale. One sentence each.
2. **Hidden coupling:** invariants, race conditions, or upstream/downstream dependencies the author may have missed.
3. **One thing to cut:** the single piece of scope whose removal would make this design 80% safer to ship.

Be specific. No throat-clearing.

DESIGN DOC:
"""
{{design_doc}}
"""