I think it's fair to say teachers have the right to be teachers without joining a teachers' union. I don't think enforcing that right "contributes to the decline" of unions. Organizational models should be able to handle that sort of freedom to choose.
To me, the question is why unions don't innovate in their organizational models and policies so that giving employees freedom to choose is moot?
I don't think it's part of an American social contract that you would be forced to tangibly support candidates who have (by some accounts) immoral positions on abortion or immigration (to pick to polarizing issues).