It is often useful to interpret statements in various modal logics using possible-world semantics. For instance "it is necessary that P" means "P is true in all possible worlds", "it is possible that P" means "P is true in some possible world", "it ought to be the case that P" means "P is true in the best possible world", "if P were true then Q would be true" means "in the closest possible world in which P is true, Q is true", etc. There are two ways to interpret these sorts of statements. Modal realists like David Lewis believe that modal statements are about real parallel universes that exist and which we may even be able to travel to in the future. Modal fictionalizers believe that possible worlds are just a useful fiction for understanding modal statements, and that "there exists a possible world in which gravity is a repulsive force" is only true in the sense that "Sherlock Holmes lives at 221B Baker Street" is true, i.e. true within its fictional context.
But my question is, is there anyone who advocates a third position, where all modal statements are trivial or meaningless? That is, someone who agrees with modal realism that modal statements really are about possible worlds, but also believes that the actual world is the only possible world. Such a person would believe that "it is necessary that P", "it is possible that P", and "it ought to be the case that P" all reduce to "P is true", and that the counterfactual conditional "if P had been true then Q would be true" reduces to the material conditional "not P or Q".
Such a position would eliminate large parts of human discourse and thought, including morality, causation, possibility and necessity, knowledge, etc. But I'd be curious to see if someone has made an argument for doing so.