The shared-viability constraint is the framework’s core principle for runaway competition, and it names what must sit above rivalry. Competition is not abolished; some things are simply placed out of its reach — no actor may externalize catastrophic risk onto everyone else, safety-critical capabilities require common verification, and strategic advantage cannot justify destroying the ecology all actors depend on. The failure it forbids is the one the scenario is built on: local viability producing global fragility, each actor rationally accelerating on its own road while the shared field becomes unsurvivable.
It belongs to a small family of realism principles. The defection test asks whether a safeguard survives another participant’s refusal; ecological predation forbids consuming a participant’s interior for advantage; the shared-viability constraint forbids consuming the field’s conditions for advantage. Stating it is easy and enforcing it is the entire difficulty — which is why it travels with verifiable pacing and mutual vulnerability, the machinery and the motive that might make rivals believe everyone else will hold the line too.