You're not taking into account two factors: union bosses don't like it because it means their fiefs shrink. Politicians/parties don't like it because it denies them a captive voting bloc. They'd actually have to do stuff like walking the length of Manhattan to get elected, rather than securing the major voting blocs by making a few backroom deals with the top unions/business leaders.
Not at all. In local politics, government employee unions and their family members get their way many times because those are the people who show up to vote, including in the primaries.
