Client diversity is often cited as the Lightning Network's defence against a bug in any one implementation, yet LND runs 84% of public nodes and holds 84% of public channel capacity, a share that has barely moved since 2023. We test what diversity buys with a natural experiment. On 26 August 2026, amid security fixes in every implementation, Core Lightning (CLN) maintainers told operators who could not upgrade to disconnect their nodes. Using 805 daily gossip snapshots, 132,360 on-chain channel closes and a client classifier validated against self-identifying aliases and against the wallet that builds each node's funding transactions, we find that 22% of CLN nodes went dark overnight, ten times the usual rate, mostly Tor-reachable ones; a third were still dark five weeks later. The rest of the network barely noticed. Reachability among all node pairs fell by 7.8 percentage points, a loss exceeded in 4 of 94 ordinary two-day windows and half that of a Tor denial-of-service wave in July, while among CLN pairs it fell by 24.5 points, more than in any ordinary window. Calibrated failure scenarios explain the containment: the blast radius depends on whether a failure reaches the hubs, not on how many nodes run the failing software. Losing ACINQ's Eclair node alone costs the surviving network four times the reachability lost with all 619 routable CLN nodes, and minority clients' reliance on LND for connectivity is what their size implies; CLN nodes in fact peer with each other more than chance. Diversity statistics that count nodes mislead; the quantity to track is which software the hubs run.

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