Iv noticed that too. No, most likely that's how it'll be for the rest of the games life
Yeah I noticed this too. I agree that they are likely going to stay there for the rest of the game's life. That's a much bigger grind than when they were limited time challenges. So these challenges are a way for people to grind to get them if they missed the limited time events.
However, for those of us who have the skins and don't want the grind, these milestones should be completed automatically. I'm a completionist but between Yoda, Obi, Maul, Luke & Leia I'm expected to get another 25,000 kills...urm, no.
