The Federation would have long experience with single-source raw materials by that point, and might have found ways to cope. Kirk was always securing unique sources of absolutely vital substances: pergium, topaline, zenite, even dilithium had to be pried from the hands of foreigners or greedy entrepreneurs rather than obtained from state-controlled sources.
Of course, such sources were never truly unique. The substances were merely specified as being rare and difficult to obtain; Kirk's efforts would have been aimed at broadening an already existing acquisition base. The same probably goes for the Gallamite duranium shipments.
Moving some production abroad might be a smart move during a conflict, really, even when shipping connections are in jeopardy. Distribution gives survivability, and the enemy might not want to expand the conflict to the incidental allies of its opponent quite yet.