Also, during the 60's the "women's movement" was growing. One interpretation is that the girls & women are left alone, without boys and men. When will "they" ever learn can also be taken as a sexist comment to the violent solutions of male-based establishment - especially when sung by women such as Joan Baez. The they points a finger or possibly blames.

I personally like the we, especially in the last verse. It is so easy to blame others. To change this in the last verse says to everyone, singler included, that we (be we human or other living organizism) are on this planet together. Yes, we go through cycles: some are immutable, but others can be changed.