What if national anthems referenced world peace, save the rain forests and acceptance in them?

Than no-one would sing them, because fuck everyone else, that's why.

Except maybe americans.

If those things were their values.

But they're not.

Some already do have quite positive messages within them, but regardless it's not like people are going to magically stop fighting, polluting, and be inclusive all of the sudden just because a nationalistic propaganda song tells them too.

I said the Pledge of Allegiance daily when I lived in the USA, but it doesn't mean I actually agree with all that is stated within it.

Then they'd cease to be national anthems. A national anthem is a critical part of nation building, just like national monuments, and is almost by definition, nationalistic in nature.

Then they'd still be a fairly empty symbol.