Sure, but that doesn't address an attack that certain precincts that vote a specific party line could be compromised. If 100 attackers at 100 precincts slit some seals than that could swing a swing state
When you have a consistent pattern of ballot spoilage, elections are not counted as normal; at that point you'd have court cases, recounts, assorted forensic attempts to verify valid votes, etc. The system is not a rigid machine - it is a set of rules for the common case, and a set of safeguards that trigger special-case handling.