Perhaps the problem is not the union per se but the members of the union.
It is quite interesting watching the decline of our pay and conditions, as the employers are quite aware, we do not stick together, that will never change.
Your very first sentence hits the nail on the head.
What/who is the union? If you can answer this question then you have almost answered your question.
Like any democracy you get what you vote for. As most people are apathetic they probably don't vote and then moan about the result.
A strong union has a strong members who stick together, and from those strong members you get strong leaders.