Decoherence is entanglement with the environment. When you become entangled with the environment, that's usually an irreversible process. There are too many moving parts in the environment so that you can't undo it. It's like scrambling an egg, hard to unscramble the egg,. In fact, literally, the entropy goes up when you become entangledwith the environment. And I think that what any sensible version of quantum mechanics would think of becoming entangled with the environments as effectively performing a measurement. Then you can break the entanglement that other subsystems have.