How would that be decentralized by definition? Swedish labor unions are not tied to specific employers, but they are usually a member of LO, the main union of unions, which I would consider centralized.
It would be decentralized in the fact that any group could create their own union without permission from anyone else. Then they could go about joining an established union or forming one of their own once they have enough people to vote for one. However they would still be able to engage in collective action to some extent.
The application wouldn't just be one large union, but thousands of small ones around the world that can talk to each other.
