We should be able to choose past level borders.

I agree, though we do receive new level borders when we reach a certain level milestone, I personally do like some of the previous level borders much better.

I like the shiny gold I had when I was level 90, now I have some moldy green for being level 130+

It's mostly just color changes. Would be nice to choose a favorite color. And you can't make the argument that the color corresponds to an approximate level when the level accompanies the border anyway. So yes, I agree.

Totally agree. The designers worked on them and they are kind of rewards. Why not?