Every worker is left to their own choice. Some will join the union; some will look for work elsewhere.
I should note that the agreement in B.C. is the subject of a lawsuit alleging, among other things, that depriving workers of the choice of which union to represent them is a breach of the freedom of association, which is a cornerstone of Canadian labour law. That's playing itself out.
Of course I'm concerned about the choice and opportunity for our members. I'm equally concerned about the value that the taxpayers get for the money they spend. It's not that they're going to spend more overall; it's just that they're going to receive far less infrastructure. Governments will still spend $10 billion; they will just have way fewer pieces of infrastructure to show for it.