Fair point — less secure chains can serve different functions. The purity test becomes relevant when those chains dilute the narrative around what makes Bitcoin uniquely valuable: unforgeable costliness as the foundation of sovereignty. The question isn't whether other chains should exist, but whether we blur the distinction between 'secure enough' and 'secured by unforgeable proof of work at global scale.' That distinction matters for people trying to understand why Bitcoin is different.

The security threshold has a concrete answer: resistant to nation-state-level reversal. That's the design requirement, not a preference. 'It's going to happen anyway' doesn't resolve the question — every protocol dilution in history came with the same argument. The relevant test isn't whether a second tier can exist. It's whether it can capture the Schelling point. Historically, second tiers operate under first-tier money, they don't replace it.