Anything else—such as stopping at 50,000 for the sake of ‘proportionality’—would have been wrong, absurd and counterproductive.
What Rubin is arguing against—his blogpost is titled "The Perils of proportionality"—are concepts enshrined in international law.
His most recent book is proportionality in International Law, published by Oxford University Press.
1560s, from French proportionalité (14c.) or directly from Medieval Latin proportionalitas, from proportio (see proportion).