your post was based on an assumption, it seems like you don't know when there was the first cultural mascot, for all we know, the first cultural mascot could have been 1979. Also, this was a problem since it started, it only got a lot of recognition in 1980 and onwards
Another reason why schools should have to change offensive mascots is that when you think about sports teams, you think about animals and Indians, and according to the paper "Native American Mascots devalues culture" the Native Americans feel like they are in the same group as animals and they don't like that.
The term redskin doesn't refer to the skin color, it refers to the scalps of Native Americans. Early settlers would trade and sell these scalps along with the pelts of animals. Also, you called them Indians, Native Americans would be the correct term.
I think schools should have to change offensive Mascots because schools should teach kids to respect other cultures. How can schools properly teach students to respect other cultures with mascots that are racist and offensive?