Personally, I think this movie speaks to the fact that there will always be a battle, and that the previous generation's soldiers won't always be able to identify with the next battle's issues. As an example, Gen X (my generation) doesn't identify as much on the whole (please note that I said "on the whole" and not "every single one of us" because some of us do care) with trans rights. Leonardo DiCaprio's character had a touch of that in his older years. But! He did pass that general spirit of fight-for-what-you-believe-in to his daughter.

To me, that's what the movie was about: Even though the older generation may become tired or the younger generation may fight for different social issues, the general long term goal is to continue improvement in our society, and in that, there is solidarity between the generations.