Katherine's purpose was to say that Elena would always choose Stefan because he is the
better man. So that when Stefan said
better man at the end they'd have a parallel. But unless Hell gave Katherine amnesia it makes zero sense for her to say that when she lived with DES in season 5 after Elena chose Damon and actually helped Stefan overcome his PTSD from being heartbroken
It was a WTF moment for me. Just like Stefan saying that Damon had literally eternity with Elena. Like how do humans have eternity?
I need to stop thinking about this stuff because it's just terrible writing. End of story
