I personally see kill tracking as a keepsake to go along with our weapon. If you are sentimental about a weapon or proud of your stats on it, you can keep it in your vault. You could pull it out every once in a while for Crucible or have some fun and try to use it after it has been sunset and see what you can do. And while yes, I think there could be an overall kill tracking system implemented, so we can look at our game history...I don't think it's a priority right now. For now, remember all the memories and fun you had with the weapon, slip it in your vault, and get excited for new ones.