 The short argument from Counterfactuals that I quoted seems deeply unQuinean. Rather than saying that possible worlds exist because they are quantified over in the best paraphrase of our theories, Lewis says they exist because they are quantified over in just one  paraphrase  of our theories. To be sure, he says this is a permissible paraphrase. On the other hand, there is vanishingly little defence of its permissibility.