Why does a justification of punishment built purely on prevention need an external limit imported from somewhere else?

D. Prevention supplies a reason to punish but no ceiling, so on its own it permits any penalty that would work

Because deterrence answers "how much punishment is useful?" and never "how much is deserved?". If a spectacular penalty for a common minor offence would suppress it, a purely preventive theory has no principled objection. The limit has to be imported — usually from desert, sometimes from proportionality as a constitutional principle — which is why almost every modern account is mixed.