[dragonwriter]

> Is there anything preventing unions from growing huge and essentially dominating entire sectors in the US?

The structure of labor law makes it just as possible for workers with a particular employer or in a particular job class with an employer to decertify—and, if they wish, simultaneously replace—a union (or for a union local to dissociate from a parent organization) as it is to organize one in the first place, which doesn't prevent parent union organizations from getting large, but does limit the success of ones that, in doing so, fail to meet the needs of their members in particular workplaces.