My napkin math says it'd be plausible to generate a pair of colliding addresses (birthday paradox etc.), but still implausible to collide with the set of existi

My napkin math says it'd be plausible to generate a pair of colliding addresses (birthday paradox etc.), but still implausible to collide with the set of existing in-use addresses. How much would the former actually matter, in the context of Yggdrasil?