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Components in series multiply their availabilities, so five dependencies at 99.9 percent give only 99.5 percent overall — worse than any individual part. Redundancy works the other way: two parallel replicas fail only when both fail, giving 1 - (1-a)^2. Combining the two is how a chain of ordinary components delivers an extraordinary service level.
Composite availability
series: A = product of component availabilities; parallel: A = 1 - (1 - a)^replicas; combined = (parallel availability)^series count
Because failures compound in series. Each additional required dependency subtracts from the total, which is the mathematical argument for graceful degradation.
Only if failures are independent. Shared power, shared control planes and correlated software bugs defeat replica maths entirely, which is why availability zones exist.