Decoys are greatly over-rated in ballistic missile systems. Sensors are so good at discriminating decoys from warheads that decoys are largely ineffective and have been for decades. This has borne out in Ukraine.

A decoy sufficiently sophisticated to look real to good sensors will have weight and characteristics that approach that of a real warhead, at which point you might as well add another warhead. Decoys only make sense if the marginal cost of adding them is low.