To be fair, the Japanese strategy also relied on heavily fortifying the territory they took and then daring the Americans to take it back. With another 6-12 months they might have had time to build up much stronger defenses. If every island was as bad as Iwo Jima/Okinawa it's an open question how long the American public would have supported the war. We ended up using nukes for multiple reasons as it was, one of which was doubt that the American public would stomach the cost of an invasion of the Japanese home islands.
At the time, the U.S. Admirals recognized that clearing the occupied islands wasn't necessary and would be ruinously expensive. Their explicit goal was to create airstrips within bomber reach of the Japanese home islands. Even with 6-12 more months to fortify, it's not obvious that the outcome wouldn't be the same, with the U.S. grabbing a few key islands and ignoring the rest.
