Those unions would have elected union reps. They’d gain an immense amount of power, and anyone on the edge of society not in a union would lose their voice — stay at home parents, small business owners, etc.

Eventually the unions would gravitate to a party system, those parties would become bipolar, and world governments would become figureheads. Unions would begin to clash, eventually forming new political bodies along union lines. Union members would question why non-union members don’t have to pay dues, and a requirement would come about that when old enough to work, it would be mandatory for everyone to pick a union.

You can see where this is going.

The only reason unions work is that they pit the power of production against the power of military strength and control. Give the unions too much power, and their leadership becomes the thing they’ve fought to resist.